Add expected output when baking a simple plugin
Further test fixes

The test fails are in part caused by assumptions related to where things
are.

Changed the way the root composer file is found to be based on the ROOT
constant rather than "the folder above the plugins dir".
Changed all the content-comparison tests to be based on the
string-compare trait.
Attempt to clear up the actual intent of the test methods, by
simplifying and renaming test methods.