test_bdb is sensitive to sys.meta_path manipulation

[jaraco]

Here's a repro not involving Setuptools showing how test_bdb is sensitive to sys.meta_path manipulation:

FROM jaraco/multipy-tox

RUN apt install -y libarchive-tools
RUN apt install -y build-essential gdb lcov pkg-config libbz2-dev libffi-dev libgdbm-dev libgdbm-compat-dev liblzma-dev libncurses5-dev libreadline6-dev libsqlite3-dev libssl-dev lzma lzma-dev tk-dev uuid-dev zlib1g-dev
RUN wget -O - https://github.com/python/cpython/archive/refs/heads/3.9.zip | bsdtar -xf -
WORKDIR cpython-3.9
RUN sh ./configure --prefix /opt/python
RUN make install
WORKDIR /
RUN /opt/python/bin/python3 -m pip install --upgrade pip
RUN /opt/python/bin/python3 -m pip uninstall -y setuptools
RUN echo "import sys; sys.meta_path.insert(0, type('NullFinder', (), dict(find_spec=lambda *args: None))())" > /opt/python/lib/python3.9/site-packages/null-finder.pth
CMD /opt/python/bin/python3 -m test_bdb

It fails with a similar error:

AssertionError: Wrong event type at expect_set item 2, got 'call'
  Expected: ('line', 3, 'tfunc_import')
  Got:      ('call', 1, '<lambda>'),        ('quit',),

Originally posted by @jaraco in #91169 (comment)

[jaraco]

I propose for consideration - perhaps this test should be configured to be skipped if sys.meta_path is not in an expected state. Or perhaps it should enforce it to be in an expected state.