cleanup: Map test files according to structure in src

For example cmd_account() is located in src/command/cmd_funcs.c.
The unit test was located in tests/unittests/test_cmd_account.c.
Let's move it to a subdirectory like tests/unittests/command/test_cmd_account.c
to correpsond to the same structure.

tests/unittests/plugins/test_plugins_disco.h

@@ -0,0 +1,7 @@
+void returns_empty_list_when_none(void** state);
+void returns_added_feature(void** state);
+void resets_features_on_close(void** state);
+void returns_all_added_features(void** state);
+void does_not_add_duplicate_feature(void** state);
+void removes_plugin_features(void** state);
+void does_not_remove_feature_when_more_than_one_reference(void** state);