testsuite: Check that testsuite paths are correctly handled

compare_dg_tests.pl's read_sum() ignores the path of an exp file up to and
including the ".../testsuite/" directory. This isn't tested in the script's
testsuite though so do that here.

Change-Id: If52dddc48c4a6c3ebfbdfeda4f0d3a8cf665dbc8
6 files changed