Log Message:
regress/show-all: migrate to regress/infra-unittests

The test framework over there makes it easier to run several independent
tests.  The previous way of squeezing all tests into a single package
Makefile did not scale well and made it invonvenient to add new tests.
In addition, there was no need to create a full-fledged package just to
test this tiny piece of the pkgsrc infrastructure, since that can be
used in categories and the top-level as well.