[3.11] gh-109615: Fix support test_copy_python_src_ignore() on WASM (GH-109970) (#109976)

gh-109615: Fix support test_copy_python_src_ignore() on WASM (GH-109970)

Not only check if src_dir exists, but look also for Lib/os.py
landmark.
(cherry picked from commit cc54bcf17b5b5f7681f52baf3acef75b995fa1fd)

Co-authored-by: Victor Stinner <vstinner@python.org>