Show parameters that caused the test failure

When an assertion fails, add the parameters to the exception message.

This produces output like:

AssertionError:
Expected: '253393.891832ws1'
but: was '253393.891832ws'

Failing parameters:
{'firstUnit': 'w',
'firstValue': 273.63017457794194,
'secondUnit': 's',
'secondValue': 926.0451345419382}