Test infrastructure documentation is lacking

[samestep]

Yesterday I opened #7637 which was meant as a forward-looking "fix" for this issue, but that PR doesn't actually fix the current issue, which is that these 13 test types defined are currently undocumented in tools/slang-test/README.md:

• SIMPLE_EX
• SIMPLE_LINE
• INTERPRET
• CPU_REFLECTION
• COMMAND_LINE_SIMPLE
• COMPARE_DXIL (currently not used in the repo AFAICT)
• CPP_COMPILER_EXECUTE
• CPP_COMPILER_SHARED_LIBRARY
• CPP_COMPILER_COMPILE
• PERFORMANCE_PROFILE
• COMPILE
• DOC (currently not used in the repo AFAICT)
• EXECUTABLE

#6611 marked some of the documented test types as deprecated, but for these 13, it is unclear which are deprecated and which are not.

[samestep]

Test options also appear to be undocumented, e.g. filecheck and filecheck-buffer.

[samestep]

Similarly the //TEST_INPUT: directive looks to be undocumented.

[samestep]

As well as other kinds of test directives like //DIAGNOSTIC_TEST: and //DISABLED_TEST:. I opened #8049 about //IGNORE_TEST:.

[samestep]

It also seems like the difference between COMPARE_COMPUTE and COMPARE_COMPUTE_EX is under-documented; the current docs just say that the latter "supports additional parameter specifications" but @saipraveenb25 mentioned that actually it also runs on e.g. Vulkan by default whereas the former doesn't.