gh-128265: Support WASI/Emscripten on PDB tests, by removing asyncio from pdb tests (#128264)

A part of `Lib/test/test_pdb.py` was previously unable to run on WASI/Emscripten
platforms because it lacked support for `asyncio`.
In fact, these tests could be rewritten without the `asyncio` framework because 
`test_pdb` tests the behavior of coroutines, which are not part of `asyncio`.

Now reliance on the availability of `asyncio` has been removed and
part of `test_pdb` that deals with coroutines working on WASI/Emscripten platforms.