Run tests on apache-parser-v2 (#7231)

We're planning on using the branch apache-parser-v2 allowing us to incrementally work on the new Apache parser and feel comfortable landing temporary test code that we don't really want in master.

The apache-parser-v2 branch is created and locked down, but neither Travis or AppVeyor are configured to run tests on it. See #7230. This PR fixes that problem.

This could probably just land in the apache-parser-v2 branch, but why unnecessarily deviate the branch from master? It doesn't hurt anything there. Once it lands, I'll get this added to the apache-parser-v2 branch too.

* Run tests on apache-parser-v2.

* add comment

* Don't run full test suite on apache-parser-v2.

appveyor.yml

@@ -7,6 +7,9 @@ environment:
 
 branches:
   only:
+    # apache-parser-v2 is a temporary branch for doing work related to
+    # rewriting the parser in the Apache plugin.
+    - apache-parser-v2
     - master
     - /^\d+\.\d+\.x$/ # Version branches like X.X.X
     - /^test-.*$/