test: always suffix tmpdir

This makes temp dir names consistent whether we run in stand-alone mode,
via `test.py` in single process, or in multi-process.

PR-URL: #28035
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>

test/common/tmpdir.js

@@ -89,11 +89,7 @@ const testRoot = process.env.NODE_TEST_DIR ?
 
 // Using a `.` prefixed name, which is the convention for "hidden" on POSIX,
 // gets tools to ignore it by default or by simple rules, especially eslint.
-let tmpdirName = '.tmp';
-if (process.env.TEST_THREAD_ID) {
-  tmpdirName += `.${process.env.TEST_THREAD_ID}`;
-}
-
+const tmpdirName = '.tmp.' + (process.env.TEST_THREAD_ID || '0');
 const tmpPath = path.join(testRoot, tmpdirName);
 
 function refresh(opts = {}) {