tests: exit from each test file with failure status

The 'suite-failed' function returns the number of failed tests in the
suite. If no tests failed, exit with status code 0. Otherwise, with
status code equal to the number of tests that failed.

This tells Automake whether a test file failed or not; but it looks like
Guile 2's extra compilation messages is interfering with Automake's
parsing of ggspec's TAP-style output. That's my working theory, anyway.
tests: successfully send TAP output to Automake

Automake now recognises individual test results within each Scheme test
file. I was wrong about Guile's extra output confusing it; turns out it
was the Makefile.am variables, which I don't exactly understand yet. But
Guile's messages are still annoying; will have to look into that.