gh-109276: libregrtest: fix worker working dir (#109313)

Fix Emscripten and WASI: start the test worker process in the Python
source code directory, where 'python.js' and 'python.wasm' can be
found. Then worker_process() changes to a temporary directory created
to run tests.

* create_worker_process() uses os_helper.SAVEDCWD as cwd.
* worker_process() uses get_temp_dir() as the parent directory for
  get_work_dir().
* Don't use plural but singual for "test" in "Run 1 test ..."
  message.
* Remove unused imports.
* Add WORK_DIR_PREFIX and WORKER_WORK_DIR_PREFIX constants.

Lib/test/libregrtest/main.py

@@ -297,7 +297,7 @@ class Regrtest:
 
         jobs = runtests.get_jobs()
         if jobs is not None:
-            tests = f'{jobs} tests'
+            tests = count(jobs, 'test')
         else:
             tests = 'tests'
         msg = f"Run {tests} sequentially"
@@ -458,7 +458,7 @@ class Regrtest:
 
     def run_tests(self, selected: TestTuple, tests: TestList | None) -> int:
         os.makedirs(self.tmp_dir, exist_ok=True)
-        work_dir = get_work_dir(parent_dir=self.tmp_dir)
+        work_dir = get_work_dir(self.tmp_dir)
 
         # Put a timeout on Python exit
         with exit_timeout():