core: MockProvider and MockProvisioner can't lock in Stop

The only reasonable usage of these methods is for them to run concurrently
with other methods, so we mustn't hold a lock to do this work. For tests
that deal with stopping, it's the test's own responsibility to deal with
any concurrency issues that arise from their StopFns running concurrently
with other mock functions.

terraform/provider_mock.go

@@ -226,8 +226,10 @@ func (p *MockProvider) Configure(r providers.ConfigureRequest) providers.Configu
 }
 
 func (p *MockProvider) Stop() error {
-	p.Lock()
-	defer p.Unlock()
+	// We intentionally don't lock in this one because the whole point of this
+	// method is to be called concurrently with another operation that can
+	// be cancelled.  The provider itself is responsible for handling
+	// any concurrency concerns in this case.
 
 	p.StopCalled = true
 	if p.StopFn != nil {

terraform/provisioner_mock.go

@@ -119,8 +119,10 @@ func (p *MockProvisioner) ProvisionResource(r provisioners.ProvisionResourceRequ
 }
 
 func (p *MockProvisioner) Stop() error {
-	p.Lock()
-	defer p.Unlock()
+	// We intentionally don't lock in this one because the whole point of this
+	// method is to be called concurrently with another operation that can
+	// be cancelled. The provisioner itself is responsible for handling
+	// any concurrency concerns in this case.
 
 	p.StopCalled = true
 	if p.StopFn != nil {