Run a single coverage command rather than two separate commands

[billsacks]

I was finding that, even with the '--append' flag, some of the coverage
results were getting overwritten in the second run, resulting in a
too-low coverage report.

This turns out to have no effect on the reported coverage on master, but
it fixes the coverage report for #90: without this change, some lines
that are covered only by unit tests are listed as uncovered in the final
coverage report (at least, I think that's what was going on).

User interface changes?: No

Fixes: None

Testing:
test removed: changed 'make coverage' operation
unit tests:
system tests:
manual testing:

[coveralls]

Coverage decreased (-0.3%) to 90.473% when pulling 2562830 on billsacks:work_on_coverage into 144f7d9 on ESMCI:master.