Fix TAP tests with Perl 5.8

Fix TAP tests with Perl 5.8

The prove program included in Perl 5.8 does not support the --ext
option, so don't use that and use wildcards on the command line instead.

Note that the tests will still all be skipped, because, for instance,
the version of Test::More is too old, but at least the regular
mechanisms for handling that will apply, instead of failing to call
prove altogether.