CI: mark user-agent tests as network
cc @jbrockmendel, inspired by https://github.com/pandas-dev/pandas/pull/43517#issuecomment-917464806. This being the source of the timeout is also supported by starting to see timeouts on windows2 after changing the split such that the io tests were included in windows2.

Hmm, actually these tests don't touch network (only use localhost) and timeout still happened, so must not be the cause Good thought!  Are you confident that the timeout(s?) are from these tests? > Good thought! Are you confident that the timeout(s?) are from these tests?

Now I am confident they are _not_ from the user-agent tests since this pr skipped them, but a timeout still occurred :) But seems likely the issue is somewhere in tests/io based on the timeouts moving to windows2 after the new split. Hoping #43545 helps