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

Fixes #233.

This way 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 kworker pegged at 70% CPU usage plus various python processes at
160%+.

Running sequentially might also make the tests more reliable (though
some of them are still consistently failing on my computer).