Avoid leaking system path from pg_available_extensions

Avoid leaking system path from pg_available_extensions

The documentation says that when extension_control_path is set to an
empty string, the default '$system' path is still assumed.  However,
get_extension_control_directories() added the system extension directory
with a NULL macro in that case.  As a result, pg_available_extensions
could expose the expanded system directory path instead of reporting
'$system' as the location.

Record the implicitly-added system directory with the '$system' macro, so
pg_available_extensions reports the documented symbolic location and does
not leak the actual system path.

Update the extension_control_path TAP test to check the reported location
directly.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Lu Feng <fnlo1995@gmail.com>
Reviewed-by: Matheus Alcantara <matheusssilv97@gmail.com>
Reviewed-by: Jim Jones <jim.jones@uni-muenster.de>
Discussion: https://postgr.es/m/357C774A-ECE9-4455-B641-315205D4D9A1@gmail.com