Minor test improvements

[StephanTLavavej]

• Mark a sporadic failure in a threading test as SKIPPED.
  • Explain how libc++ speculatively implemented LWG-3343 (thanks @CaseyCarter).
• Print exit codes in both decimal and hex (thanks to Sir Walrus on Discord for the suggestion).
  • Verified that this prints Exit Code: 3 (0x3) for an abort() and Exit Code: 3221225477 (0xC0000005) for a null dereference.
• Cite <sstream>, <syncstream>: Buffer types assume that allocated pointers are not modified by users #4268 now that @frederick-vs-ja has analyzed these failures.
• Explain how libc++ speculatively implemented an old proposed resolution for LWG-3645. According to the ultimately accepted wording, their test is bogus and MSVC's STL is behaving correctly here.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.