Libstdc++: Combine duplicated tests for <atomic>
Redundant C++20-specific tests for <atomic> are removed and merged into the C++0x tests.
The C++20-specific tests for <atomic> in libstdc++ were almost identical to the C++0x tests. Now that all tests can run for any -std option, and run with -std=gnu++20 by default, the C++20 tests are redundant. The C++20-specific parts are merged into the C++0x tests, and the C++20 tests are removed to reduce duplication.
In Details
The libstdc++ testsuite contains multiple copies of the same test, differentiated only by C++ standard version.
This commit merges types_std_c++20.cc and types_std_c++0x.cc. A toolchain developer might not immediately know that the tests are identical, or that the build system can run all tests for all standards versions.
For Context
The C++ standard library includes a test suite to verify that the library functions correctly. Some tests were duplicated for different C++ standard versions, but this is no longer necessary. This commit removes the duplicated tests to simplify the test suite. This change reduces redundancy and makes the test suite easier to maintain.