tests: Make tests return non-zero status on failure

All the test were being run properly, and failures could be locally
observed by reading console output. However, the travis-ci script was
assuming that the tests would return a non-zero status code on failure,
which was not happening.

In order to correct this, the following line was added to the bottom of
tests/Test.ml:

  Pa_ounit_lib.Runtime.summarize ()

As the name implies, this will summarize the results of the test run and
in addition return a non-zero status if any tests failed.

Author: seliopou

test/Test.ml

@@ -101,3 +101,5 @@ module RoundTrip = struct
       (prop_roundtrip parse marshal))
 
 end
+
+Pa_ounit_lib.Runtime.summarize ()