tests: Run valgrind with absolute path to suppressions file

So far we are providing the suppressions file with a relative
path to valgrind. This apparently doesn't work on some distros
like Ubuntu and its derivates. Providing the absolute path fixes
the problem.
Signed-off-by: NShotaro Gotanda <g.sho1500@gmail.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

tests/Makefile.am

@@ -495,7 +495,7 @@ TESTS_ENVIRONMENT = \
 
 VALGRIND = valgrind --quiet --leak-check=full --trace-children=yes \
 	--trace-children-skip="*/tools/virsh","*/tests/commandhelper" \
-	--suppressions=$(srcdir)/.valgrind.supp
+	--suppressions=$(abs_srcdir)/.valgrind.supp
 valgrind:
 	$(MAKE) check VG="$(LIBTOOL) --mode=execute $(VALGRIND)"