When doing quteproc.open_path, by default the test didn't wait until the page
was loaded. This caused unintentional race conditions which e.g. caused
dirbrowser tests to fail on OS X:
https://travis-ci.org/The-Compiler/qutebrowser/jobs/133730001
Now instead we wait by default, unless wait=False is passed to open_path() or
open_url().
Otherwise if we only run a keypress test, the fake clipboard won't be
activated and thus the subsequent :yank-selected would write to the real
clipboard.
