Fix test_that_stbt_templatematch_rejects_invalid_parameters on OS X

33e8e356 added an `abspath` before passing the template filename to
`stbt.match`. On my OS X system, where `/var` is a symlink to
`/private/var` and `TMPDIR` is inside `/private/var`, the template
filename was showing as `/private/var` but `$PWD` was just `/var`
so the test's check was failing.