[compiletest-related cleanups 3/7] Make the distinction between sources root vs test suite sources root in compiletest less confusing

[jieyouxu]

Reference for overall changes: #136437
Part 3 of 7 of the compiletest-related cleanups PR series.

Summary

• Remove --src-base compiletest in favor of new flags --src-root and --src-test-suite-root which more accurately conveys the intent. --src-base previously actually meant --src-test-suite-root and has caused multiple confusions.
• Use --src-root to have bootstrap directly feed source root path to compiletest, instead of doing a hacky directory parent search heuristic (find_rust_src_root) that somehow returns an Option<PathBuf>.

Review advice

Best reviewed commit-by-commit.

r? bootstrap