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().
--cachedir="" doesn't work because the quotes are not processed (as they
would be by the shell) and the cachedir is set to ./"" (that is a
directory with two double quotes as name). The correct start parameter
is thus --cachedir=, which correctly fails when the fix is reverted.
