compare_tests: distinguish c-c++-common results by tool

When compare_tests compares both C and C++ tests in c-c++-common, they
get the same identifier, so expected differences in results across
languages become undesirably noisy.

This patch adds tool identifiers to tests, so that runs by different
tools are not confused by the compare logic.

It also fixes a bug in reporting differences, that would attempt to
print an undefined fname (the definitions are in subshell loops), and
adjusts the target insertion to match tabs in addition to blanks after
colons.


for  contrib/ChangeLog

	* compare_tests: Add tool to test lines.  Match tabs besides
	blanks to insert tool and target.  Don't print undefined fname.