Refactor regression tests to use a custom program instead of pg_regress.

Refactor regression tests to use a custom program instead of pg_regress.

Using pg_regress was awkward, because it used a dummy .sql file for each
test that just launched the test program. Also, it added a dependency;
the driver doesn't require PostgreSQL sources to build, just libpq
headers, but the pgxs program is not usually shipped with libpq headers.

The new "runsuite" program runs all the test programs given on the command
line, compares the results with the expected output, and produces
TAP-compatible output. It can be run stand-alone, as the TAP output is
human-readable, or with the perl 'prove' program which gives more pretty
output.