Bug 1461383 - fix test_build.py for local builds; r=chmanchester,f=mshal

test_build.py fails on local builds with messages like:

make[4]: Entering directory '/tmp/tmp65BjCH'
make[4]: *** No rule to make target 'buildid.h'.  Stop.
make[4]: Leaving directory '/tmp/tmp65BjCH'
/home/froydnj/src/gecko-dev.git/config/faster/rules.mk:76: recipe for target '/tmp/tmp65BjCH/buildid.h' failed
make[3]: *** [/tmp/tmp65BjCH/buildid.h] Error 2
make[3]: *** Waiting for unfinished jobs....
make[4]: Entering directory '/tmp/tmp65BjCH'
make[4]: *** No rule to make target 'source-repo.h'.  Stop.
make[4]: Leaving directory '/tmp/tmp65BjCH'
/home/froydnj/src/gecko-dev.git/config/faster/rules.mk:76: recipe for target '/tmp/tmp65BjCH/source-repo.h' failed
make[3]: *** [/tmp/tmp65BjCH/source-repo.h] Error 2
make[3]: Leaving directory '/tmp/tmp65BjCH/faster'
Makefile:155: recipe for target 'faster' failed

The tests pass in automation, however, because automation always defines
the rules for buildid.h and source-repo.h in the toplevel Makefile.in.
For local builds, however, those rules are not defined to avoid build
churn.  Let's ensure that the necessary rules are defined during testing
as well.

Makefile.in

@@ -33,8 +33,8 @@ DIST_GARBAGE = config.cache config.log config.status* config-defs.h \
 ifndef MOZ_PROFILE_USE
 # Automation builds should always have a new buildid, but for the sake of not
 # re-linking libxul on every incremental build we do not enforce this for
-# developer builds.
-ifneq (,$(MOZ_AUTOMATION)$(MOZ_BUILD_DATE))
+# developer builds.  Tests always need a new buildid as well.
+ifneq (,$(MOZ_AUTOMATION)$(MOZ_BUILD_DATE)$(TEST_MOZBUILD))
 buildid.h source-repo.h: FORCE
 endif
 endif