Audit {`check-run-results`, `regex-error-pattern`, `error-pattern`, `check-stdout`, `normalize-*`, `dont-check-compiler-*`, `run-rustfix`, `rustfix-only-machine-applicable`, `forbid-output`} family of directives

[jieyouxu]

I don't know about other contributors, but I have to look at the source implementation (or trial-and-error) every time I see or try to use one of these compiletest directives. IMO, we should audit the design of these directives, and possibly revamp them entirely.

Concrete confusions:

• check-run-results check both run stderr and stdout and puts them into snapshot files (on bless), then compares the subsequent run stderr and stdout against the snapshot.
• error-pattern doesn't only check stderr (whose stderr? compiler? run?), it can also check stdout (or both??) depending on check-stdout, dont-check-compiler-*, and also it can check also compiler stderr or stdout I think??
• normalize-* (that is not normalize-stdout or normalize-stderr) I believe can simultaneously apply to {compiler,run} {stderr,stdout}.
• run-rustfix will run rustfix and try to apply all non-placeholder suggestions, including non-machine-applicable ones like MaybeIncorrect ones.
• rustfix-only-machine-applicable is like run-rustfix but only tries to apply MachineApplicable suggestions.
• forbid-output is like error-pattern but named completely differently. I don't remember which output pattern of {compiler,run}x{stderr,stdout} it is forbidding.

EDIT:

• regex-error-pattern is like error-pattern but accepts a regex...