Make sure that we join paths correctly.

We were simply concatenating a pair of paths together but this can
cause problems if the second path is not relative. This could occur if
the OUT_DIR environment variable has been set.

Test: ./test/testrunner/testrunner.py --host -j40
Change-Id: I8b3fa817cdb114e39f9cf33a0c2e73e1a53f7e99
diff --git a/test/testrunner/env.py b/test/testrunner/env.py
index e015d74..278980f 100644
--- a/test/testrunner/env.py
+++ b/test/testrunner/env.py
@@ -165,7 +165,7 @@
   ART_PHONY_TEST_HOST_SUFFIX = "64"
   _2ND_ART_PHONY_TEST_HOST_SUFFIX = "32"
 
-HOST_OUT_EXECUTABLES = ('%s/%s') % (ANDROID_BUILD_TOP,
+HOST_OUT_EXECUTABLES = os.path.join(ANDROID_BUILD_TOP,
                                     get_build_var("HOST_OUT_EXECUTABLES"))
 os.environ['JACK'] = HOST_OUT_EXECUTABLES + '/jack'
 os.environ['DX'] = HOST_OUT_EXECUTABLES + '/dx'