In order to support a variety of values for the --with-scriptlet-shell
configure flag, pmtest has to be aware of what kind of path was passed,
be it an absolute path or a fragment for a path lookup. For absolute
paths, leave the path alone. For fragments, search the PATH environment
var for the resolved path to the binary. In both cases, join the
resultant path to the root directory defined for the test, not a
pre-determined bin directory.

Fixes FS#31552.

With-contribution-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Allan McRae <allan@archlinux.org>