ggspec: remove stub definition
Add user story
text-normal: improve the runtime message function

The text-normal runtime message function, which is intended to be the
default test run output printing function, now prints more relevant and
focused messages. E.g., it now prints all details of assertion failures,
but nothing if an assertion succeeds.

The output is now more powerful, at the cost of more complexity.

Have also switched the test suite over to use the default, text-normal.