tests: add some tests to bundle and illustrate current issues

[ReadWriteDenied]

Wrote some tests to better describe and indicate current issues (not all fail). These new tests run twice, once to verify the behavior of the test itself (asyncio with default policy) and once with the 'GlibEventLoopPolicy' to verify it's conformance. Maybe this can also help to identify (a) proper fix(es).

Note that currently 3 tests fail, combining (something in line of) #5 and #9 fixes these (see https://github.com/ReadWriteDenied/asyncio-glib/actions/runs/667968855).