Add check for path regexps

This CL adds a check to //infra/config/branch/cq_cfg_presubmit.py to
check the path regexps in cq.cfg to make sure there is at least one file
that they could match with.

This CL also fixes a few typos which already exist in cq.cfg, as a
result of this missing checking.

Bug: 902507
Change-Id: I535d6671e4598eda27a0919ef14404a228398ea7
Reviewed-on: https://chromium-review.googlesource.com/c/1334940
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612756}