MDL-81457 core: Mark SSL test as skipped, not incomplete

The incomplete test is intended for tests which are written but whose
code does not yet exist.

https://docs.phpunit.de/en/9.6/incomplete-and-skipped-tests.html

lib/dml/tests/pgsql_native_moodle_database_test.php

@@ -409,7 +409,8 @@ class pgsql_native_moodle_database_test extends \advanced_testcase {
             // ... or fail with SSL not supported.
             $this->assertStringContainsString($pgconnerr, $e->debuginfo);
             $this->assertStringContainsString('server does not support SSL', $e->debuginfo);
-            $this->markTestIncomplete('SSL not supported.');
+            $this->markTestSkipped('Postgres server does not support SSL. Unable to complete the test.');
+            return;
         }
 
         try {