[PATCH 1 of 6 V2] tests: omit -p for external diff via extdiff extension for portability

[Danek Duvall]

For what it's worth, I've been patching the Solaris-delivered copy of
extdiff to just call GNU diff directly.  If that's the general expectation
(and I don't think that's unreasonable), then it's just a matter of making
sure that the right diff gets executed.  Extdiff sets the default options
to include -N and -p, anyway, and I didn't want to change those.

I couldn't think of a reasonable way of making that change suitable for
pushing upstream, so I've just kept it local.

Danek