TST: Remove subset of singleton fixtures
Broken off of #24769 where we have learned that some test behavior depends on whether or not a fixture is being used.

I claim that this is another point in favor of not using fixtures if there is a regular-python alternative (in this case, a "function").  Whenever with-fixture behavior is different from without-fixture behavior, it is definitely the latter that better represents user runtime environments.  That is what we should be testing.