Add pkgrepo tests for CentOS

Ubuntu ones will follow soon.
Fix pkgrepo tests when python file is run by itself on CLI
Remove grains requirement

I had this code here in an early draft of this test, but the grains are
no longer needed here as the stuff that requires grains has been moved
out of the test function and into the SLS files that are run by the
test.
Add an assert to ensure that the state.sls return dict isn't empty
Fix #8773: allow syntax error state.foo in requisites with a warning.
Merge pull request #8820 from terminalmage/tests

Add pkgrepo tests for CentOS
Merge pull request #8828 from UtahDave/develop

fix typo
Merge pull request #8823 from regilero/requisites-fix

Fix #8773: allow syntax error state.foo in requisites with a warning.
Don't need the plus sign here
fix pylint errors.  Need space after each arg