[contrib] validate_failures.py: Avoid testsuite aliasing

[contrib] validate_failures.py: Avoid testsuite aliasing

This patch adds tracking of current testsuite "tool" and "exp"
to the processing of .sum files.  This avoids aliasing between
tests from different testsuites with same name+description.

E.g., this is necessary for testsuite/c-c++-common, which is ran
for both gcc and g++ "tools".

This patch changes manifest format from ...
<cut>
FAIL: gcc_test
FAIL: g++_test
</cut>
... to ...
<cut>
		=== gcc tests ===
Running gcc/foo.exp ...
FAIL: gcc_test
		=== gcc Summary ==
		=== g++ tests ===
Running g++/bar.exp ...
FAIL: g++_test
		=== g++ Summary ==
</cut>.

The new format uses same formatting as DejaGnu's .sum files
to specify which "tool" and "exp" the test belongs to.

contrib/ChangeLog:

	* testsuite-management/validate_failures.py: Avoid testsuite
	aliasing.