clang-tidy: issues in NotificationsTest.cpp

[debruce]

1. do not use namespace using-directives; use using-declarations instead (line 22)
2. destructor probably needs override at line 36
3. magic numbers used at lines 47, 63, 85, 109, 130
4. Complains about 'auto source_filter = source;' being a needless copy at line 65, similar complaints at line 111
5. invalid case style for num_messages (line 66), notificationSource (line 68 and 114), num_messages (line 112)
6. not initialized: rx_queue_mtx (line 73), message (line 94), rx_queue_mtx (line 118)
7. Bogus if-then-else complaint hidden inside a gtest macro (linee 81, 89, 126, 134)
8. is unused: message (line 86 and 131)