fix test pollution in test_assertrewrite

[asottile]

originally reproduced with this pollution set:

testing/test_assertrewrite.py::TestEarlyRewriteBailout::test_pattern_contains_subdirectories
testing/test_assertrewrite.py::TestRewriteOnImport::test_remember_rewritten_modules

[nicoddemus]

Great find!

originally reproduced with this pollution set:

How are you finding these?

[asottile]

Great find!

originally reproduced with this pollution set:

How are you finding these?

I found this one while fixing the other one. basically get the test list and bisect the list with the falling test (maybe I'll make a video about it, or write some automation -- I wrote a tool for this for testify based tests previously)

[nicoddemus]

basically get the test list and bisect the list with the falling test

OK thanks. That's what I do too basically. Never got around writing a plugin to automate doing that though.