Fix `held_by_thread` in `no-std` to return instead of panicing

Fix `held_by_thread` in `no-std` to return instead of panicing

Our `no-std` locks simply panic if a lock cannot be taken as there
should be no lock contention in a single-threaded environment.
However, the `held_by_thread` debug methods were delegating to the
lock methods which resulted in a panic when asserting that a lock
*is* held by the current thread.

Instead, they are updated here to call the relevant `RefCell`
testing methods.