[libcxx] [test] Add LIBCPP_ASSERT_NOEXCEPT/LIBCPP_ASSERT_NOT_NOEXCEPT, remove an unused variable.

test/support/test_macros.h
For convenience/greppability, add macros for libcxx-specific static_asserts about noexceptness.

(Moving the definitions of ASSERT_NOEXCEPT/ASSERT_NOT_NOEXCEPT isn't technically necessary
because they're macros, but I think it's better style to define stuff before using it.)

test/std/utilities/tuple/tuple.tuple/tuple.apply/apply.pass.cpp
There was a completely unused `TrackedCallable obj;`.

apply() isn't depicted with conditional noexcept in C++17.

test/std/utilities/tuple/tuple.tuple/tuple.apply/make_from_tuple.pass.cpp
Now that we have LIBCPP_ASSERT_NOEXCEPT, use it.

Fixes D27622.

llvm-svn: 289264