When running tests with --save-dir use the local out/test directory (#18562)

In general this should be easier to find than hunting around in the
system temp dir, and it won't conflict with other users on the same
system also running tests with `--save-dir`.

Author: sbc100

site/source/docs/getting_started/test-suite.rst

@@ -198,7 +198,7 @@ emits debug output and intermediate files (the files go in
 
 
 You can also specify ``--save-dir`` to save the temporary directory that the
-test runner uses into **/tmp/emscripten_test/**.  This is a test suite-specific
+test runner uses into **/out/test/**.  This is a test suite-specific
 feature, and is useful for inspecting test outputs as well as temporary files
 generated by the test.  By default, the temporary directory will be cleaned
 between each test run, but you can add ``--no-clean`` to avoid this.

test/common.py

@@ -583,7 +583,7 @@ def setUp(self):
           self.temp_files_before_run.append(os.path.normpath(os.path.join(root, filename)))
 
     if EMTEST_SAVE_DIR:
-      self.working_dir = os.path.join(self.temp_dir, 'emscripten_test')
+      self.working_dir = path_from_root('out/test')
       if os.path.exists(self.working_dir):
         if EMTEST_SAVE_DIR == 2:
           print('Not clearing existing test directory')