Moved File parse request handling and diagnostic extraction flow into
python to simplify flow and allow easier addition of new parse request
handlers such as semantic highlighter.
Refactored base_test to patch separate vimsupport functions instead of
the whole module, and interfering the test results afterwards.
Added new tests for diagnostic sign place/unplace and error/warning
count extraction API.
assert_has_calls is not enough to check if a call didn't raise a
warning. We also need to check the number of calls. This is done by
creating a subclass of MagicMock implementing the
assert_has_exact_calls method.
