make check: Run stbt-camera tests sequentially, and not at all if stbt-camera disabled

Fixes https://github.com/drothlis/stb-tester/issues/233

This implementation seems less error-prone than remembering to add
"skip" at the beginning of each new test case in test-camera.sh.

This also runs the stbt-camera tests sequentially. Running them in
parallel was *killing* the responsiveness of other apps on my computer,
with various python processes at 160%+ CPU usage.

Running sequentially might also make the tests more reliable (though
some of them are still consistently failing on my computer).
Makefile: Require enabling "stbt camera" support explicitly

There are a number of outstanding issues with the "stbt camera"
self-tests, so to avoid inconveniencing users who want to run "make
check" but aren't interested in the camera support, this has to be
enabled explicitly with "make enable_stbt_camera=yes".