Make Mandatory Test Failure More Prominent

[shawnbrown]

Currently, when a mandatory test fails, the test runner exists with the following:

$ python -m datatest 
.......F
==========================================================
FAIL: <test name here>
<traceback here>
datatest.error.DataError: mandatory test failed, stopping 
early: data does not satisfy object requirement:
 Invalid('Some Invalid Code')
----------------------------------------------------------
Ran 8 tests in 1.764s

FAILED (failures=1)

The summary at the end should, more prominently, indicate that not all the tests were run:

$ python -m datatest 
.......F
==========================================================
FAIL: test_<test name here>
<traceback here>
datatest.error.DataError: mandatory test: data does not 
satisfy object requirement:
 Invalid('Some Invalid Code')
----------------------------------------------------------
Ran 8 tests in 1.764s

MANDATORY TEST FAILED, STOPPING EARLY