If the call to requires() doesn't precede the filesystem check, we get the following situation:
1. ./python Lib/test/regrtest.py test_foo # test needs urlfetch, not enabled, so skipped
2. ./python Lib/test/regrtest.py -u urlfetch test_foo # test runs
3. ./python Lib/test/regrtest.py test_foo # test runs (!)

By moving the call to requires() *before* the filesystem check, the fact that fetched files are cached on the local disk becomes an implementation detail, rather than a semantics-changing point of note.