Test: Clarify "non-standard extension"

We're currently saying "non-standard extension" in several test files.

https://github.com/microsoft/STL/blob/3f76681e18fa51e1441c9150099b8010c1d18e1c/tests/std/tests/GH_000431_copy_move_family/test.cpp#L436
https://github.com/microsoft/STL/blob/3f76681e18fa51e1441c9150099b8010c1d18e1c/tests/std/tests/GH_000431_equal_family/test.cpp#L586
https://github.com/microsoft/STL/blob/3f76681e18fa51e1441c9150099b8010c1d18e1c/tests/std/tests/GH_000431_equal_memcmp_is_safe/test.compile.pass.cpp#L506
https://github.com/microsoft/STL/blob/3f76681e18fa51e1441c9150099b8010c1d18e1c/tests/std/tests/GH_000431_iter_copy_move_cat/test.compile.pass.cpp#L419
https://github.com/microsoft/STL/blob/3f76681e18fa51e1441c9150099b8010c1d18e1c/tests/std/tests/VSO_0000000_string_view_idl/test.cpp#L249

This is seemingly redundant, because every extension can hardly to be considered "standard extension". Perhaps it would be clearer to say "non-conforming/conformant" extension.

• For conversion from function pointers to void*, I believe this extension is non-conforming. As [conv.ptr]/2 only allows implicit conversion from pointer to object types to cv void*.
• For basic_string_view::_Copy_s(), I believe this extension is conforming because it's under an _Ugly name, and we can just say "an extension".

There're also comments in <yvals_core.h> saying "non-Standard extension(s)". But as they refer to removed deprecation messages, I think we shouldn't modify them.

https://github.com/microsoft/STL/blob/3f76681e18fa51e1441c9150099b8010c1d18e1c/stl/inc/yvals_core.h#L1463-L1468
https://github.com/microsoft/STL/blob/3f76681e18fa51e1441c9150099b8010c1d18e1c/stl/inc/yvals_core.h#L1484

It seems to be a https://github.com/microsoft/STL/labels/good%20first%20issue to improve the comments in these test files.

Review the five linked test files at the referenced lines and compare each comment's use of "non-standard extension" with the accompanying C++ standard wording. Clarify the comments according to whether the extension is conforming, non-conforming, or simply an extension, while leaving the noted yvals_core.h comments unchanged. Done means the test comments use precise, consistent terminology.