tests: Trace re-executing processes with valgrind

A lot of our tests re-execute themeselves after loading their
mock library. This, however, makes valgrind sad because currently
we do not tell it to trace the process after exec().
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

tests/Makefile.am

@@ -470,7 +470,8 @@ TESTS_ENVIRONMENT =				\
   $(VG)
 
 
-VALGRIND = valgrind --quiet --leak-check=full \
+VALGRIND = valgrind --quiet --leak-check=full --trace-children=yes \
+	--trace-children-skip="*/tools/virsh","*/tests/commandhelper" \
 	--suppressions=$(srcdir)/.valgrind.supp
 valgrind:
 	$(MAKE) check VG="libtool --mode=execute $(VALGRIND)"