compare_dg_tests.pl: Prefix test name with testsuite and .exp file name

This will help report regressions or start bisections .

The .exp file name is still listed at the beginning of the section,
the reason it was only present there so far was to make reports more
compact.

This change showed that an .exp file didn't have the expected name in
sum-2.txt, so it is updated as needed, along with the expected
results.

Change-Id: I7571a59d291d77f6e7db51b598c1dc98e0c1ae7d
6 files changed