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/command/test_cmd_disconnect.c

@@ -12,7 +12,7 @@
 #define CMD_DISCONNECT "/disconnect"
 
 void
-clears_chat_sessions(void** state)
+cmd_disconnect__updates__clears_chat_sessions(void** state)
 {
     chat_sessions_init();
     roster_create();