ssl: Add connection and reload tests for key passphrases

ssl: Add connection and reload tests for key passphrases

ssl_passphrase_command_supports_reload was not covered by the SSL
testsuite,  and connection tests after unlocking secrets with the
passphrase was also missing.  This adds test coverage for reloads
of passphrase commands as well as connection attempts which tests
the different codepaths for Windows and non-EXEC_BACKEND builds.

Author: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/5F301096-921A-427D-8EC1-EBAEC2A35082@yesql.se