Make 'gmx insert-molecules' tests test output

The tests now actually test the output for regression, and provide an
easy way to check the output files using the reference data framework.
Convert the tests to use CommandLineTestBase for a more uniform
approach.  Add a title to the test system to avoid generating random
titles for the output.

Change-Id: I3b1e6242922bf24ab4c254e6e0a53531af84c9df