Rollup merge of #136441 - jieyouxu:cleanup-is-rustdoc, r=compiler-errors

[`compiletest`-related cleanups 1/7] Cleanup `is_rustdoc` logic and remove a useless path join in rustdoc-json runtest logic

Reference for overall changes: rust-lang/rust#136437
Part **1** of **7** of the *`compiletest`-related cleanups* PR series.

### Summary

- Don't match on path when we already have test suite names.
- Remove a useless path join.

r? bootstrap (or compiler)

Author: jieyouxu