We should document more clearly env vars used between bootstrap <-> compiletest <-> test suites

[jieyouxu]

Location

bootstrap <-> compiletest <-> testsuites

Summary

In tests/run-make (for example, but the lack of docs isn't limited to this particular test suite) we assume a bunch of env vars being available, but AFAIK there isn't any docs on which env vars are assumed available between the bootstrap -> compiletest -> test suites pipeline and for what reasons. It'd be nice to have any kind of docs on them.

The current state is that:

• bootstrap sets up a bunch of env vars, which are made available to compiletest
• compiletest assumes bootstrap-set env vars are available
• compiletest sets up a bunch of env vars, often conditionally available to specific test suites
• tests inside test suites assume compiletest-set env vars are available.

[jieyouxu]

Example:

• COMPILETEST_NEEDS_ALL_LLVM_COMPONENTS -> COMPILETEST_REQUIRE_ALL_LLVM_COMPONENTS: compiletest: clarify COMPILETEST_NEEDS_ALL_LLVM_COMPONENTS error #125710