fixup! test: use unique tmpdirs for each test

Co-Authored-By: Rich Trott <[email protected]>

test/common/README.md

@@ -910,7 +910,7 @@ The first time `refresh()` runs,  it adds a listener to process `'exit'` that
 cleans the temporary directory. Thus, every file under `tmpdir.path` needs to
 be closed before the test completes. A good way to do this is to add a
 listener to process `'beforeExit'`. If a file needs to be left open until Node.js
-completes, use a child process and call `refresh` only in the parent.
+completes, use a child process and call `refresh()` only in the parent.
 
 ## WPT Module