Use installed postgresql.conf.sample for GUC sanity TAP test

The current code looks for the sample file in the source directory, but
it seems better to test against the installed sample file.

Backpatch to release 15 where the test was introduced.

Discussion: https://postgr.es/m/73eea68e-3b6f-5f63-6024-25ed26b52016@dunslane.net

Reviewed by Tom Lane, Alvaro Herrera, Michael Paquier.

Author: adunstan

src/test/modules/test_misc/t/003_check_guc.pl

@@ -33,10 +33,9 @@
      ORDER BY 1");
 my @not_in_sample_array = split("\n", lc($not_in_sample));
 
-# TAP tests are executed in the directory of the test, in the source tree,
-# even for VPATH builds, so rely on that to find postgresql.conf.sample.
-my $rootdir     = "../../../..";
-my $sample_file = "$rootdir/src/backend/utils/misc/postgresql.conf.sample";
+# use the sample file from the temp install
+my $share_dir = $node->config_data('--sharedir');
+my $sample_file = "$share_dir/postgresql.conf.sample";
 
 # List of all the GUCs found in the sample file.
 my @gucs_in_file;