squash! make check: Don't run stbt-camera tests if stbt-camera disabled

[I think this way is easier to read than the previous commit's
implementation.]

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).