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/plugins/test_callbacks.h

@@ -1,2 +1,7 @@
-void returns_no_commands(void** state);
-void returns_commands(void** state);
+#ifndef TESTS_TEST_CALLBACKS_H
+#define TESTS_TEST_CALLBACKS_H
+
+void plugins_get_command_names__returns__no_commands(void** state);
+void plugins_get_command_names__returns__commands_when_added(void** state);
+
+#endif