test: cover user-mode apply atomicity and non-blocking KILL

db: TestSetSessionUserModesIsAtomic installs a trigger that rejects the
is_oper UPDATE after the is_wallops UPDATE has run, proving the '+w-o'
partial-failure the old independent-UPDATE loop exhibited is gone.

ircserver: TestDisconnectDoesNotBlockOnUnresponsiveVictim drives
Disconnect against a net.Pipe victim that never reads and requires the
call to return promptly; verified to fail against the synchronous
implementation. Its counterpart asserts the notification is still
delivered and the socket still closed.

internal/db/export_test.go

@@ -58,3 +58,19 @@ func (database *Database) Close() error {
 
 	return nil
 }
+
+// ExecForTest runs a raw statement against the test
+// database.  Tests use it to install SQLite triggers that
+// force a specific write to fail, so that the atomicity of
+// multi-statement helpers can be exercised.
+func (database *Database) ExecForTest(
+	ctx context.Context,
+	query string,
+) error {
+	_, err := database.conn.ExecContext(ctx, query)
+	if err != nil {
+		return fmt.Errorf("exec for test: %w", err)
+	}
+
+	return nil
+}