Tests: allow some time for D-Bus signals to arrive
Tests: allow some time for D-Bus signals to arrive

The unit test test_signals_other_manager() sends one signal over D-Bus
and expects to read it back as soon as the main loop is re-entered, but
this is not a given.
Modify the test to wait for up to two seconds for the signal to arrive.

plotters/accounts-sso.libaccounts-glib,plotters/accounts-sso.libaccounts-glib,plotters/accounts-sso.libaccounts-glib