Add better coverage support under tox

For those in the community who don't use run_tests.sh but want
tox to have coverage support this small change should help.  It
adds coverage erase, combine, html reporting.  It also
fixes the coverage unit tests in two ways.  The first is
adding the locking like normal unit tests. The second
is adding substring regular expression matching to pick
a subset of tests.  This brings it up to closer par with
run_tests.sh.

Change-Id: If9ea229425dc9a191fa7c5cc073cf5da412a69b7
Fix a wrong comment in the code

The docstring in the ExtensionManager class indicates a test file as
guide file, but the path reported is wrong.

Change-Id: I1b95d591cee6da3cc760c365c88ae1809973b98c
Merge "Fix a wrong comment in the code"
Merge "Add better coverage support under tox"