How to run unit tests only in a Hybris project?

You should use ant unittests and not ant unit tests:

ant unittests -Dtestclasses.extensions=myext

Note

Running simple unit tests exclusively is not so easy whenever someone uses somewhere Registry.getApplicationContext() in the code under test!

In fact, Registry.getApplicationContext() starts a Hybris instance. If that happens to you, you need to eliminate that particular call to Registry.getApplicationContext() with a better class design and/or mocks.

This is good information. However, in my opinion, even running the unit tests for a single extension is still too much. Unit tests are supposed to be FAST! I should be able to run a single unit test method from within my IDE if I choose to. The whole concept of "red-green testing" is lost if I have to wait for a bunch of non-relevant unit tests to run every time I want to test my refactored code.

Because these tests rely on a runtime environment, there are NO unit tests in Hybris. There are only integration tests because they all rely on a running Hybris system to be executed.