Fix lock file error with test/runner and EMCC_DEBUG mode. (emscripten-core#26110)

`cleanup_emscripten_temp()` was run at the beginning of tests and caused
the `emscripten.lock` file that was created earlier to be removed. Then
when test was finished it attempted to delete that lock file again.

Author: brendandahl

test/runner.py

@@ -711,8 +711,10 @@ def set_env(name, option_value):
 
   check_js_engines()
 
-  # Remove any old test files before starting the run
-  cleanup_emscripten_temp()
+  # Remove any old test files before starting the run. Skip cleanup when we're running in debug mode
+  # where we want to preserve any files created (e.g. emscripten.lock from shared.py).
+  if not (shared.DEBUG or common.EMTEST_SAVE_DIR):
+    cleanup_emscripten_temp()
   utils.delete_file(common.flaky_tests_log_filename)
 
   browser_common.init(options.force_browser_process_termination)