Revert "ruby: Run tests one at a time, instead of in parallel."

It seems the default behaviour of rake is to run tests sequentially, and
not in parallel (there are separate gems to achieve that behaviour).

Hence just invoke "rake test" to run all the available tests at once.

This reverts commit 8f30c3c3f8.

ruby/run-ruby-tests

@@ -18,10 +18,4 @@
 
 set -e
 
-# Run them one at a time, otherwise rake runs them in parallel (which
-# is bound to fail because they all use a single test image file).
-
-for f in t/tc_*.rb; do
-    echo $RAKE test "$@" TEST="$f"
-    $RAKE test "$@" TEST="$f"
-done
+$RAKE test "$@"