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.