What are fixtures in programming?

I think PHP-unit tests have very good explaining of this:

One of the most time-consuming parts of writing tests is writing the code to set the world up in a known state and then return it to its original state when the test is complete. This known state is called the fixture of the test.

Also Yii documents described fixtures test in a good shape:

Automated tests need to be executed many times. To ensure the testing process is repeatable, we would like to run the tests in some known state called fixture. For example, to test the post creation feature in a blog application, each time when we run the tests, the tables storing relevant data about posts (e.g. the Post table, the Comment table) should be restored to some fixed state.

Here's a simple example of fixtures test:

<?php
use PHPUnit\Framework\TestCase;

class StackTest extends TestCase
{
    protected $stack;

    protected function setUp()
    {
        $this->stack = [];
    }

    protected function tearDown()
    {
        $this->stack = [];
    }

    public function testEmpty()
    {
        $this->assertTrue(empty($this->stack));
    }

    public function testPush()
    {
        array_push($this->stack, 'foo');
        $this->assertEquals('foo', $this->stack[count($this->stack)-1]);
        $this->assertFalse(empty($this->stack));
    }

    public function testPop()
    {
        array_push($this->stack, 'foo');
        $this->assertEquals('foo', array_pop($this->stack));
        $this->assertTrue(empty($this->stack));
    }
}
?>

This PHP unit test has functions with names setUp and tearDown so that before running your tests you setup your data and once finished you can restore them to the initial state.

Exactly to that topic, JUnit has a well explained doc. Here is the link!

The related portion of the article is:

Tests need to run against the background of a known set of objects. This set of objects is called a test fixture. When you are writing tests you will often find that you spend more time writing the code to set up the fixture than you do in actually testing values.

To some extent, you can make writing the fixture code easier by paying careful attention to the constructors you write. However, a much bigger savings comes from sharing fixture code. Often, you will be able to use the same fixture for several different tests. Each case will send slightly different messages or parameters to the fixture and will check for different results.

When you have a common fixture, here is what you do:

Add a field for each part of the fixture Annotate a method with @org.junit.Before and initialize the variables in that method Annotate a method with @org.junit.After to release any permanent resources you allocated in setUp For example, to write several test cases that want to work with different combinations of 12 Swiss Francs, 14 Swiss Francs, and 28 US Dollars, first create a fixture:

public class MoneyTest {
    private Money f12CHF;
    private Money f14CHF;
    private Money f28USD;

    @Before public void setUp() {
    f12CHF= new Money(12, "CHF");
    f14CHF= new Money(14, "CHF");
    f28USD= new Money(28, "USD");
    }
}

I think you're referring to test fixtures:

The purpose of a test fixture is to ensure that there is a well known and fixed environment in which tests are run so that results are repeatable. Some people call this the test context.

Examples of fixtures:

• Loading a database with a specific, known set of data
• Erasing a hard disk and installing a known clean operating system installation
• Copying a specific known set of files
• Preparation of input data and set-up/creation of fake or mock objects

(source: wikipedia, see link above)

Here are also some practical examples from the documentation of the 'Google Test' framework.

The term fixture varies based on context, programing language or framework.

1. A known state against which a test is running

One of the most time-consuming parts of writing tests is writing the code to set the world up in a known state and then return it to its original state when the test is complete. This known state is called the fixture of the test. PHP-Unit documentation

A test fixture (also known as a test context) is the set of preconditions or state needed to run a test. The developer should set up a known good state before the tests, and return to the original state after the tests. Wikipedia (xUnit)

2. A file containing sample data

Fixtures is a fancy word for sample data. Fixtures allow you to populate your testing database with predefined data before your tests run. Fixtures are database independent and written in YAML. There is one file per model. RubyOnRails.org

3. A process that sets up a required state. 

A software test fixture sets up the system for the testing process by providing it with all the necessary code to initialize it, thereby satisfying whatever preconditions there may be. An example could be loading up a database with known parameters from a customer site before running your test. Wikipedia