test_plan_advice: Set TAP test priority 50 in meson.build.

test_plan_advice: Set TAP test priority 50 in meson.build.

Since this runs the main regression tests, it can take some time to
complete. Therefore, it's better to start it earlier, as we also do
for the main regression test suite.

Author: Matheus Alcantara <matheusssilv97@gmail.com>
Discussion: http://postgr.es/m/1095d3fe-a6eb-4d83-866e-649d6f369908@gmail.com