To ensure that tests that modify cookies but don't finish cleanly don't
leave cookie values that interfere with subsequent tests, we need to
clear the cookie storage in between each test that's run.
Testing: Can't test the test runner, just have to watch the patterns in
our test runs.
Fixes: #40668Fixes: #40672Fixes: #40673Fixes: #40674Fixes: #40695Fixes: #40697
Signed-off-by: Josh Matthews <josh@joshmatthews.net>