Remove incorrect C++20 check from test/CMakeLists.txt

[alexezeder]

This check disables entire branch of tests declaration unconditionally because CXX_STANDARD is not defined there. But even if we use CMAKE_CXX_STANDARD there, these tests should work for standards ≥20 too, IMHO.

Here are the tests:

• std-format-test (now removed)
• noexception-test
• nolocale-test
• compile-error-test

std-format-test does not compile after enabling it, since it's either outdated or should still be disabled. @vitaut, could you explain what should be done with it?

[vitaut]

std-format-test is incompatible with C++20 so the check should be preserved but possibly moved to a place where it only applies to this test.

[alexezeder]

I've disabled std-format-test with standards ≥ C++20, but there are still errors in format header.

[alexezeder]

@vitaut, I suggest merging this PR to some temporary branch, where you can apply needed changes for std-format-test since I'm not sure that I can adapt it to the latest version of the library. Of course, you could guide me here by discussions, but it would be much faster for you to fix the remaining problems. What do you think about that?
Or we can simply disable std-format-test for now. In this case, I will create an issue to enable it again.

[vitaut]

I removed std-format-test since the API it has been testing has already been merged into the standard. Please rebase.

[alexezeder]

Rebased.

[vitaut]

Thank you