Fix inconsistency in SQLite test's age

This was added in a8a4cee3 and removed in 5df15c (both in PR #841).

a8a4cee3: Fix computation of test's age (SQLite)

Check against "creation_time" to compute the test"s age instead of
"test_start_time" as this is done with MySQL and PostgreSQL.

lib/Zonemaster/Backend/DB/SQLite.pm

@@ -128,7 +128,7 @@ sub recent_test_hash_id {
 
     my $dbh = $self->dbh;
     my ( $recent_hash_id ) = $dbh->selectrow_array(
-        "SELECT hash_id FROM test_results WHERE fingerprint = ? AND test_start_time > DATETIME('now', ?)",
+        "SELECT hash_id FROM test_results WHERE fingerprint = ? AND creation_time > DATETIME('now', ?)",
         undef, $fingerprint, "-$age_reuse_previous_test seconds"
     );