[libc++][test] Merge test files for `mdspan::at` (#199330)
`libcxx/test/libcxx/containers/views/mdspan/mdspan/assert.at.pass.cpp`
caused build bot failures for
- sanitizer-aarch64-linux-bootstrap-asan
- sanitizer-aarch64-linux-bootstrap-hwasan
- sanitizer-aarch64-linux-bootstrap-msan
It's not yet clear why current mechanisms don't work for these builds.
`TEST_HAS_NO_EXCEPTIONS` should have been working.
Also remove one unnecessary `static` and use `std::string_view(e.what())
== "mdspan"`.
