test_web.py: fix memory leak when run with --until-failure

test_web.py: fix memory leak when run with --until-failure

The Fake*Node classes in test/common.py were accumulating share data in
a class-level dictionary, which persisted from one test run to the next.
As a result, running test_web.py over and over (with trial's
--until-failure feature) made this dictionary grow without bound,
eventually running out of memory.

This fix moves that dictionary into the FakeClient built fresh for each
test, so it doesn't build up. It does the same thing for "file_types",
which was much smaller but still lived at the class level.

Closes #1729