tests: standardize naming convention

Use a behavior-driven naming convention for unit tests.

Functions are now named using the pattern: [unit]__[verb]__[scenario]
The __ works as a semantic separator.

Examples:
* jid_create__returns__null_from_null()
* cmd_connect__shows__usage_when_no_server_value

Benefits:
* Easy to find all tests associated with a specific function using the
  mandatory prefix.
* Test output in CI now explicitly describes the unit, the expected
  outcome, and the scenario being tested.

Also disabled keyhandlers tests due to missing code in src/ui.

tests/unittests/xmpp/test_chat_session.h

@@ -1,4 +1,9 @@
-void returns_false_when_chat_session_does_not_exist(void** state);
-void creates_chat_session_on_recipient_activity(void** state);
-void replaces_chat_session_on_recipient_activity_with_different_resource(void** state);
-void removes_chat_session(void** state);
+#ifndef TESTS_TEST_CHAT_SESSION_H
+#define TESTS_TEST_CHAT_SESSION_H
+
+void chat_session_get__returns__null_when_no_session(void** state);
+void chat_session_recipient_active__updates__new_session(void** state);
+void chat_session_recipient_active__updates__replace_resource(void** state);
+void chat_session_remove__updates__session_removed(void** state);
+
+#endif