Fix case where no example name could be found.

Fix by no longer using minispec-metadata's #spec_name, but
just extracting description from example.name.

Not sure why minispec-metadata's #spec_name was failing, but the
new test case here demonstrates it failing. Turns out minitest/spec
itself simply doesn't keep track of the original description.
minispec-metadata was trying, but failed for some reason. We won't
try to keep track of it, but instead we'll try to extract it from
what minitest/spec does have, "test_NNNN_original example desc".

I didn't know what to do if (in some future version of minitest?),
the example name wasn't of this format, if I should raise or what.
I just used the original full "test_NNNN_x" or whatever in that case.