plperl and pltcl installcheck targets

The attached patch creates installcheck targets for plperl and pltcl
(plpython laready has one). This will help in getting buildfarm to test PLs.

Is it worth rearranging things for plpython so that it follows the same
test layout as the other 2 (i.e. a test subdir with all the test files
and a script called runtest that does the work)? Especially if we bring
in other PLs as has been discussed, some standard might be useful. Maybe
a README in src/pl ?

I'm not sure that the tests currently provide very good coverage
(especially plperl) but getting them run automatically might provide
some extra incentive on that.

cheers

andrew