Makefile: change default value of TESTTIMEOUT from 1 hour to 15 min

The whole test suite takes 6 minutes on my laptop. It takes less than 30
minutes on most buildbots. The TESTTIMEOUT is the timeout for a single test
file.

Makefile.pre.in

@@ -974,7 +974,7 @@ $(LIBRARY_OBJS) $(MODOBJS) Programs/python.o: $(PYTHON_HEADERS)
 TESTOPTS=	$(EXTRATESTOPTS)
 TESTPYTHON=	$(RUNSHARED) ./$(BUILDPYTHON) $(TESTPYTHONOPTS)
 TESTRUNNER=	$(TESTPYTHON) $(srcdir)/Tools/scripts/run_tests.py
-TESTTIMEOUT=	3600
+TESTTIMEOUT=	900
 
 # Run a basic set of regression tests.
 # This excludes some tests that are particularly resource-intensive.