How can I load this file into an NUnit Test?

You could specify in the properties of the file to be copied to the output folder and inside the unit test:

string text = File.ReadAllText(Path.Combine(TestContext.CurrentContext.TestDirectory, "TestData", "126.txt"));

As an alternative you could embed this file as a resource into the test assembly and then:

var assembly = Assembly.GetExecutingAssembly();
using (var stream = assembly.GetManifestResourceStream("ProjectName.Tests.IntegrationTests.TestData.126.txt"))
using (var reader = new StreamReader(stream))
{
    string text = reader.ReadToEnd();
}

If you do not want the files as ManifestResources, but just as file on the system. See Trouble with NUnit when determining the assembly's directory for more info and this answer in particular

Also interesting is the info from NUnit https://bugs.launchpad.net/nunit-vs-adapter/+bug/1084284/comments/3

But here is the quick info:

Path.Combine(TestContext.CurrentContext.TestDirectory, @"Files\test.pdf")

Where Files\test.PDF is just a file in your test project, with build action content and copy to output directory copy if newer

All credits go out to the people in the other post, but took me a while to find that answer, and that is the reason why i am adding the answer to this post.