Add mock call assertions to TestWorkflowEnvironment

[sebneira]

What was changed

Added AssertCalled, AssertNotCalled and AssertNumberOfCalls to TestWorkflowEnvironment to enable a straightforward way to assert for activity calls within workflows.

They have been implemented following the same approach as https://pkg.go.dev/github.com/stretchr/testify/mock#Mock.

Why?

Asserting how an activity has been called from within a workflow (e.g. storing/retrieve data from a db) can be cumbersome without assertions on how mocks have been called.

The current best approach are:

• storing the arguments in variables to later assert them
• asserting within the mocks handler function (which leads to very long stacks when the assertion fails)

The testify library provides AssertCalled, AssertNotCalled and AssertNumberOfCalls for this purpose, making the assertions straight-forward to understand and debug.

Checklist

1. How was this tested:
   Two unit tests have been implemented to test these cases.

2. Any docs updates needed?
   In https://docs.temporal.io/docs/go/how-to-test-workflow-definitions-in-go/, Test_SimpleWorkflow_ActivityParamCorrect could be updated to read

func (s *UnitTestSuite) Test_SimpleWorkflow_ActivityParamCorrect() {
        s.env.OnActivity(SimpleActivity, mock.Anything, mock.Anything).Return("", nil)
        s.env.ExecuteWorkflow(SimpleWorkflow, "test_success")

        s.env.AssertCalled(s.T(), SimpleActivity, mock.Anything, "test_success")
        s.env.AssertNumberOfCalls(s.T(), SimpleActivity, 1)
        s.True(s.env.IsWorkflowCompleted())
        s.NoError(s.env.GetWorkflowError())
}

[CLAassistant]

All committers have signed the CLA.

[cretz]

@sikian - This looks good to me, thanks! Mark it as ready for review instead of draft and I'll gladly approve and merge it.

[sebneira]

Thanks @cretz!
Wanted to have the team check this before you guys got to it but happy to see you're ok merging as is.

[cretz]

Thanks! I approved. I'll leave it open for a bit to see if anyone else wants to look.