Cancellation of external workflow needs test coverage

[Spikhalskiy]

Is your feature request related to a problem? Please describe.

User-facing "external" workflow cancelation API and implementation that are not covered by any unit test.

See

ExternalWorkflowStub#cancel
SyncWorkflowContext#cancelWorkflow

https://coveralls.io/builds/57953701/source?filename=temporal-sdk%2Fsrc%2Fmain%2Fjava%2Fio%2Ftemporal%2Finternal%2Fsync%2FSyncWorkflowContext.java#L1037