Refactor: remove key parameter from testvar methods (#7040)

## What changed?
<!-- Describe what has changed in this PR -->
Remove `key` parameter from `testvar` methods.

## Why?
<!-- Tell your future self why have you made these changes -->
It turn out to be confusing for people on how to use optional `key`
parameter. @bergundy got a great idea to remove this parameter and use
different instance of `tv` when more than one entity is needed.

## How did you test it?
<!-- How have you verified this change? Tested locally? Added a unit
test? Checked in staging env? -->
Run all tests. Refactoring affects test code only.

## Potential risks
<!-- Assuming the worst case, what can be broken when deploying this
change to production? -->
No risks.

## Documentation
<!-- Have you made sure this change doesn't falsify anything currently
stated in `docs/`? If significant
new behavior is added, have you described that in `docs/`? -->
Yes, updated testing docs.

## Is hotfix candidate?
<!-- Is this PR a hotfix candidate or does it require a notification to
be sent to the broader community? (Yes/No) -->
No.

common/testing/testvars/any.go

@@ -25,11 +25,14 @@
 package testvars
 
 import (
+	"time"
+
 	"github.com/pborman/uuid"
 	commonpb "go.temporal.io/api/common/v1"
 	failurepb "go.temporal.io/api/failure/v1"
 	"go.temporal.io/server/common/payload"
 	"go.temporal.io/server/common/payloads"
+	"google.golang.org/protobuf/types/known/durationpb"
 )
 
 type Any struct {
@@ -80,6 +83,10 @@ func (a Any) ApplicationFailure() *failurepb.Failure {
 	}
 }
 
+func (a Any) InfiniteTimeout() *durationpb.Duration {
+	return durationpb.New(10 * time.Hour)
+}
+
 func (a Any) RunID() string {
 	return uuid.New()
 }