tests for stbt screenshot/templatematch/tv should test source version

...not the installed version of stbt.

The tests execute `stbt-screenshot` (note the dash) to pick up the
program from the source folder (which I'll call "$srcdir").
`stbt-screenshot` was then executing `stbt run`. Because `make check`
sets `PATH=$srcdir:$PATH`, during the tests this would find
`$srcdir/stbt`. But `$srcdir/stbt` has hard-coded paths to
`$prefix/libexec/stbt/stbt-run` and thus the test ends up running
the *installed* version of `stbt`.

If `stbt-screenshot` etc. call `stbt-run` instead of `stbt run`, it will
work correctly in both cases: from the installed location, and from the
unit tests.

This commit also fixes two unrelated issues:

+ The GStreamer plugin wasn't being rebuilt when you ran `make
  check-integrationtests` instead of `make check`.

+ Set up the PATH for running tests in `tests/run-tests.sh` instead of
  in the `make check-integrationtests` recipe. This is so that you can
  run `tests/run-tests.sh` directly without having to manually set up
  the correct environment.
README: Minor cleanup; link to website instructions for building from source