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.c

@@ -7,7 +7,7 @@
 #include "plugins/plugins.h"
 
 void
-returns_no_commands(void** state)
+plugins_get_command_names__returns__no_commands(void** state)
 {
     plugins_init();
     GList* commands = plugins_get_command_names();
@@ -16,7 +16,7 @@ returns_no_commands(void** state)
 }
 
 void
-returns_commands(void** state)
+plugins_get_command_names__returns__commands_when_added(void** state)
 {
     plugins_init();
     PluginCommand* command1 = g_new0(PluginCommand, 1);