Fix two src/test/modules Makefiles

commit_ts and test_pg_dump were declaring targets before including the
PGXS stanza, which meant that the "all" target customarily defined as
the first (and therefore default target) was not the default anymore.
Fix that by moving those target definitions to after PGXS.

commit_ts was initially good, but I broke it in commit 9def031b;
test_pg_dump was born broken, probably copying from commit_ts' mistake.

In passing, fix a comment mistake in test_pg_dump/Makefile.

Backpatch to 9.6.

Noted by Tom Lane.