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

[jieyouxu]

Better yet, in bootstrap maybe use a pattern like an env var set with possibly fixed/declared keys that is gradually populated, so that when it's time to call compiletest from bootstrap, the env var set will do cmd.env() in a centralized place. Between compiletest and test suites it's more tricky, but a similar thing can be done to make it easier to track what possible collection of env vars tests can receive.

Additionally, we need to enforce better discipline on what env vars are propagated through, because atm it's a wild west of sometimes just letting through whatever env var the user writes.