Introduce PG_TEST_TIMEOUT_DEFAULT for TAP suite non-elapsing timeouts.

Introduce PG_TEST_TIMEOUT_DEFAULT for TAP suite non-elapsing timeouts.

Slow hosts may avoid load-induced, spurious failures by setting
environment variable PG_TEST_TIMEOUT_DEFAULT to some number of seconds
greater than 180.  Developers may see faster failures by setting that
environment variable to some lesser number of seconds.  In tests, write
$PostgreSQL::Test::Utils::timeout_default wherever the convention has
been to write 180.  This change raises the default for some briefer
timeouts.  Back-patch to v10 (all supported versions).

Discussion: https://postgr.es/m/20220218052842.GA3627003@rfd.leadboat.com