Tests: Skip tests that we don't have the appropriate dependencies for

The goal is to allow ./run-tests -i to work against installed versions
of stb-tester, without requiring any additional dependencies to be
installed beyond stb-tester's existing runtime dependencies.

This should help with automated testing of the packaged versions of
stb-tester as implemented in "extra/fedora/test-packaging.sh", although
further work is required and this script will not yet succeed.
stbt-batch.d/templates/index.html: Remove unused code

Unused since b784a841.
stbt batch report: Ignore invalid "extra-columns" lines

These lines can be added by the user's classify or pre/post-run hooks,
so we should be robust to garbage.
stbt batch: summary report: List pass/fail/error counts per testcase

The code is ugly, but it works. I had initially tried to use
underscore.js's `_.groupBy` but I can't use underscore.js because of
https://github.com/asimihsan/tablequeryjs/issues/20
stbt batch: summary report: Hide subtotals by default; click to show