Small TAP improvements

Here's a couple of small patches I came up with while doing some related
work on TAP tests.

The first makes the argument for $node->config_data() optional. If it's
not supplied, pg_config is called without an argument and the whole
result is returned. Currently, if you try that you get back a nasty and
cryptic error.

The second changes the new GUCs TAP test to check against the installed
postgresql.conf.sample rather than the one in the original source
location. There are probably arguments both ways, but if we ever decided
to postprocess the file before installation, this would do the right thing.

cheers

andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com