b=352305 use DIRS not TOOL_DIRS for test-related targets

from discussion with bsmedberg on why
tools/test-harness/xpcshell-simple/example was not running correctly

tools/test-harness/xpcshell-simple/Makefile.in

@@ -47,8 +47,8 @@ MODULE		= test_harness_xpcshell_simple
 
 # Here's how you let the build system know there are tests in the
 # "example" folder:
-ifdef ENABLE_TESTS
-TOOL_DIRS  += example
+ifdef ENABLE_TESTS
+DIRS  += example
 endif
 
 include $(topsrcdir)/config/rules.mk