It's cleaner to use a generic function to test that a sequence of
strings is found in a specific order rather than fetching and
comparing string locations in test case code.

Also make sure that string occurrences can't overlap when looking for
a specific number of occurrences of the same string.

TEST=angle_unittests

Change-Id: I8ca66c73c7aaa5be8469ded466f51d97a36c801b
Reviewed-on: https://chromium-review.googlesource.com/793041Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>