main: exit with status code number of failed tests
suite-{passed,failed,skipped}: results accessor functions

So that users aren't exposed to the internal structure of results.
suite: predicate variable names end with '?'
suite-add-option: add tests

And move from the 'ggspec' file to the 'lib.scm' file for better reuse.
ggspec-spec.scm: test all output functions in one suite

Thanks to the 'suite-add-option' function, we can pass in options to
suites before evaluating them; this lets us set up a single suite and
test it with different output options.