Have the scripts get the tests directory from $0.

Have the scripts get the tests directory from $0.

That means we don't have to cook up a way to pass $(srcdir) to them - we
run them from $(srcdir), so they can pick it up from there.  If you run
them by running tests/TESTrun.sh from the command line, that will also
work; we assume that it's not found from $PATH.

TESTrun.sh and the other scripts run scripts with a path that includes
${testsdir}, so the other scripts are run with a path (they're not
intended to be run from the command line - you're supposed to use
TESTrun.sh, even if you're just running one test).