Disable fallback on cannonical results file in pg_regress

In GPDB, platform_expectfile is used for determining
ORCA/planner/resgroup expect files, wheras in upstream that is not the
case and it is based on the underlying platform. Thus, it is unnecessary
and confusing to compare against default answer file even when
platform_expect file exists. It gets confusing because the below block
chooses the best expect file based on the number of lines in diff file.
Co-authored-by: NEkta Khanna <ekhanna@pivotal.io>

src/test/regress/pg_regress.c

@@ -1665,8 +1665,16 @@ results_differ(const char *testname, const char *resultsfile, const char *defaul
 	/*
 	 * fall back on the canonical results file if we haven't tried it yet and
 	 * haven't found a complete match yet.
+	 *
+	 * In GPDB, platform_expectfile is used for determining ORCA/planner/resgroup
+	 * expect files, wheras in upstream that is not the case and it is based on
+	 * the underlying platform. Thus, it is unnecessary and confusing to compare
+	 * against default answer file even when platform_expect file exists. It gets
+	 * confusing because the below block chooses the best expect file based on
+	 * the number of lines in diff file.
 	 */
 
+#if 0
 	if (platform_expectfile)
 	{
 		snprintf(cmd, sizeof(cmd),
@@ -1688,7 +1696,7 @@ results_differ(const char *testname, const char *resultsfile, const char *defaul
 			strlcpy(best_expect_file, default_expectfile, sizeof(best_expect_file));
 		}
 	}
-
+#endif
 	/*
 	 * Use the best comparison file to generate the "pretty" diff, which we
 	 * append to the diffs summary file.