run_remote_tests: create a symlink to the last test that completed

Having this symlink is extremely useful if your running the same
test over and over again.

BUG=n0ne
TEST=Verified that the symlink is there.

Change-Id: I625a48ce4348082db05951ffd67929687ad461cc

Review URL: http://codereview.chromium.org/6543021

run_remote_tests.sh

@@ -79,6 +79,7 @@ function cleanup() {
      [[ ${RAN_ANY_TESTS} -eq ${FLAGS_FALSE} ]]; then
     rm -rf "${TMP}"
   else
+    ln -sf "${TMP}" /tmp/run_remote_tests.latest
     echo ">>> Details stored under ${TMP}"
   fi
   stop_ssh_agent