Allow buildbot to pass extra test flags to regrtest (specifically -r) via the environment.

Makefile.pre.in

@@ -550,7 +550,7 @@ $(LIBRARY_OBJS) $(MODOBJS) Modules/$(MAINOBJ): $(PYTHON_HEADERS)
 # generated bytecode.  This is sometimes a very shy bug needing a lot of
 # sample data.
 
-TESTOPTS=	-l
+TESTOPTS=	-l $(EXTRATESTOPTS)
 TESTPROG=	$(srcdir)/Lib/test/regrtest.py
 TESTPYTHON=	$(RUNSHARED) ./$(BUILDPYTHON) -E -tt
 test:		all platform