Fix unorderable types error when sorting slow tests

[sirex]

Part, where slow tests are printed, where not cevered with tests. Added tests
for that.

In rare cases, when time of two test cases are the same, sorted function lookes
to the next element of tuple, and next element is test instance which is not
orderable. And that rare case it gives following error:

    Traceback (most recent call last):
      File "CoverageTestRunner.py", line 312, in <module>
          run()
      File "CoverageTestRunner.py", line 303, in run
          result = runner.run()
      File "CoverageTestRunner.py", line 265, in run
          for secs, test in sorted(result.timings)[-10:]:
    TypeError: unorderable types: FooTests() < FooTests()

The fix was to always sort only by time, not by whole tuple.

[mgedmin]

LGTM