Change path to hiddenapi for run-tests

Some test runners don't seem to have ANDROID_HOST_OUT in their PATH.
Make it explicit.

Test: art/test.py --host -b -r -t 674-hiddenapi
Change-Id: Ie9c8ad7738eaeeb06909397fc056ddd57fbc7a70
diff --git a/test/run-test b/test/run-test
index bde3aac..a453f22 100755
--- a/test/run-test
+++ b/test/run-test
@@ -113,9 +113,10 @@
 fi
 export ZIPALIGN
 
-# If hiddenapi was not set by the environment variable, assume it is in the path.
+# If hiddenapi was not set by the environment variable, assume it is in
+# ANDROID_HOST_OUT.
 if [ -z "$HIDDENAPI" ]; then
-  export HIDDENAPI="hiddenapi"
+  export HIDDENAPI="${ANDROID_HOST_OUT}/bin/hiddenapi"
 fi
 
 info="info.txt"