[blinkpy] Add PRESUBMIT tests for `wpt lint`

Write a PRESUBMIT_test.py for LayoutTests/external/PRESUBMIT.py to check
if it can catch WPT lint errors properly. This PRESUBMIT_test.py will
run whenever LayoutTests/external/PRESUBMIT.py is modified.

Besides, we also need to test the basic functionalities of `wpt lint`
when we roll in a new version, so a new PRESUBMIT.py is added to
blinkpy/third_party/wpt which calls the PRESUBMIT_test.py above.
(https://www.chromium.org/developers/how-tos/depottools/presubmit-scripts)

Bug: 819349
Change-Id: I48766db3dd98f91bf57f86e944f35019d2033ac5
Reviewed-on: https://chromium-review.googlesource.com/1081944
Reviewed-by: Philip Jägenstedt <[email protected]>
Commit-Queue: Philip Jägenstedt <[email protected]>
Cr-Commit-Position: refs/heads/master@{#563643}
3 files changed