Rollup merge of #60719 - varkor:xpy-test-folder, r=Mark-Simulacrum
Rollup merge of #60719 - varkor:xpy-test-folder, r=Mark-Simulacrum

Allow subdirectories to be tested by x.py test

Fixes https://github.com/rust-lang/rust/issues/60718.

As far as I can tell, multiple `--test-args` flags are ignored (only the first is respected), so if you specify a subdirectory, you won't also be able to filter using `--test-args`. If you don't specify a subdirectory, `--test-args` will continue working as usual, so this is strictly an improvement on the current state of affairs.

aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust