Make test failures fatal.

This was made non-fatal in the past because we want to be able to have
`libc++-test: libc++-test-target libc++-test-host`, and still run both
if the first fails, but in reality it's just hiding test failures from
the buildbot.

Change-Id: I762e7da7393a72b82f72751404dfab50919e0a74

test.mk

@@ -1,4 +1,4 @@
 .NOTPARALLEL:
 default:
-	-python $(LIT) -sv --param android_mode=$(LIT_MODE) $(LIT_ARGS) \
+	python $(LIT) -sv --param android_mode=$(LIT_MODE) $(LIT_ARGS) \
         $(ANDROID_BUILD_TOP)/external/libcxx/test