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.
diff --git a/Makefile.pre.in b/Makefile.pre.in
index 6514bf8..5c90daf 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -974,7 +974,7 @@
 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.