[NFC][libc++][format] Enable unit tests.

The GCC failures were partly fixed in D124103.

The format functions tests are fixed on GCC but they require a huge
amount of RAM (>10 GB). This fails with parallel testing in the CI. It
can be solved by splitting the test, but GCC-12 will be released shortly
and might fix the memory usage in these tests. Therefore these tests
remain disabled.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D124335