More flexible API for xvg testing

More flexible API for xvg testing

Make checking xvg files with CommandLineTestHelper::setOutputFile()
explicit.  Also unify setOutputFileNoTest() to just have a single
method.  Add a parameter to the method for the caller to provide a
matcher to use for the file contents, and implement three matchers for
exact, xvg, and no matching.

Unify the reference data XML structure for all types of output files,
and make the xslt files aware of this structure.

Rename checkStringBlock() to checkTextBlock() for consistency.

Change-Id: I17d1ca2f5b4f7bad4b41c418c3ecd0d04f962abc