SourceLayout: Cleanup unit tests (#324)

SourceLayout: Cleanup unit tests (#324)

Polish up and add some structure to the unit tests in
src/Makefile.am. Documenting current best practice for new test
additions and shuffling of lines around to make the tests easily
managed in future, consistent with that practice.

There are two functional changes amongst the non-logic changes;

1) testUfs and testRock src/fs/ tests are updated to using
   automake conditionals to wrap the entire set of automake lists.
   Not just the part adding the test binary to those built. This
   reduces the compiler work creating dependency information for
   them in builds where they are not going to be used.

2) testRefCount has missing LDFLAGS list added

Also, shuffling of most SOURCES lists into nodist_*_SOURCES to
comply with the documented practice is left to followup work
pruning those lists down.