Install TAP test infrastructure so it's available for extension testing.

When configured with --enable-tap-tests, "make install" will now install
the Perl support files for TAP testing where PGXS will find them.
This allows extensions to rely on $(prove_check) even when being built
out-of-tree.  Back-patch to 9.4 where we first started to support TAP
testing, to reduce the number of cases extension makefiles need to
consider.

Craig Ringer

Discussion: <CAMsr+YFXv+2qne6xJW7z_25mYBtktRX5rpkrgrb+DRgQ_FxgHQ@mail.gmail.com>