Make it possible to cancel workflows by ID when testing

[recht]

What was changed

Added a function in TestWorkflowEnvironment to allow canceling arbitrary workflows, in line with SignalWorkflowByID and QueryWorkflowByID

Why?

This makes it possible to cancel child executions and test the behavior of doing so.

[CLAassistant]

All committers have signed the CLA.

[Quinn-With-Two-Ns]

Thanks for the contribution, can you please add some unit tests to verify this change? I would like to see a test like this

sdk-go/internal/internal_workflow_testsuite_test.go

Line 570 in 5621b7f

func (s *WorkflowTestSuiteUnitTest) Test_WorkflowCancellation() {

for the new API and a test that cancels a child workflow using the new API.