proposal: separate databases for contrib module testing

I'd like to change the way we set the CONTRIB_TESTDB name for contrib
modules. so that each module doesn't wipe out the previous module's test
db. The reason is that this will let us test upgrading them using
pg_upgrade much more easily. Not testing this is a significant hole in
the pg_upgrade testing regime.

This can be achieved by a fairly simple change in Makefile.global.in
along these lines:

ifneq ($(MODULE_big),)
CONTRIB_TESTDB = contrib_regression_$(MODULE_big)
else
ifneq ($(MODULES),)
CONTRIB_TESTDB = contrib_regression_$(MODULES)
else
CONTRIB_TESTDB = contrib_regression
endif
endif

plus some changes in the dblink tests / results that rely on the
database name.

The downside is that this involves in increase in space of 6.5Mb to
7.5Mb per module. That doesn't seem huge in these days when a standard
commodity hard drive is 500Gb and up.

Thoughts?

cheers

andrew