Support 'make test TESTSUITE=CodeGen', etc.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@81011 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/Makefile b/test/Makefile
index 13cbfe6..3fb103e 100644
--- a/test/Makefile
+++ b/test/Makefile
@@ -2,7 +2,11 @@
 include $(LEVEL)/Makefile.common
 
 # Test in all immediate subdirectories if unset.
+ifdef TESTSUITE
+TESTDIRS := $(TESTSUITE:%=$(PROJ_SRC_DIR)/%)
+else
 TESTDIRS ?= $(PROJ_SRC_DIR)
+endif
 
 ifndef TESTARGS
 ifdef VERBOSE