Split 002_pg_dump.pl into two test files.

Split 002_pg_dump.pl into two test files.

Add a new test script 006_pg_dump_compress.pl, containing just
the pg_dump tests specifically concerned with compression, and
remove those tests from 002_pg_dump.pl.  We can also drop some
infrastructure in 002_pg_dump.pl that was used only for these tests.

The point of this is to avoid the cost of running these test
cases over and over in all the scenarios (runs) that 002_pg_dump.pl
exercises.  We don't learn anything more about the behavior of the
compression code that way, and we expend significant amounts of
time, since one of these test cases is quite large and due to get
larger.

The intent of this specific patch is to provide exactly the same
coverage as before, except that I went back to using --no-sync
in all the test runs moved over to 006_pg_dump_compress.pl.
I think that avoiding that had basically been cargo-culted into
these test cases as a result of modeling them on the
defaults_custom_format test case; again, doing that over and over
isn't going to teach us anything new.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/3515357.1760128017@sss.pgh.pa.us