Use pg_reload_conf() to reload the configuration file in regression test.

Use pg_reload_conf() to reload the configuration file in regression test.

Previously the regression test ran pg_ctl reload command for that purpose.
However in the environment where the path to PostgreSQL binaries is not set,
the regression test always failed because pg_ctl could not be found.
To reload the configuration file even in that environment, this commit
changes the regression test so that it uses pg_reload_conf function, instead.