Revert "Borrow Linux test job for xDS tests"

This reverts commit a041056e71.

test/kokoro/linux.cfg

@@ -14,10 +14,10 @@
 # Config file for Kokoro (in protobuf text format)
 
 # Location of the continuous shell script in repository.
-build_file: "grpc-node/packages/grpc-js-xds/scripts/xds.sh"
-timeout_mins: 360
+build_file: "grpc-node/test/kokoro.sh"
+timeout_mins: 60
 action {
   define_artifacts {
-    regex: "github/grpc/reports/**"
+    regex: "github/grpc-node/reports/**/sponge_log.xml"
   }
-}
+}