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/xmpp/test_muc.h

@@ -0,0 +1,9 @@
+int muc_before_test(void** state);
+int muc_after_test(void** state);
+
+void test_muc_invites_add(void** state);
+void test_muc_remove_invite(void** state);
+void test_muc_invites_count_0(void** state);
+void test_muc_invites_count_5(void** state);
+void test_muc_room_is_not_active(void** state);
+void test_muc_active(void** state);