Deal with golint warning with private executeShutdownFunc

```
  Error: pkg/action/release_testing.go:60:57: unexported-return: exported method Run returns unexported type action.executeShutdownFunc, which can be annoying to use (revive)
  func (r *ReleaseTesting) Run(name string) (ri.Releaser, executeShutdownFunc, error) {

```

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

pkg/action/hooks.go

@@ -46,14 +46,14 @@ func (cfg *Configuration) execHook(rl *release.Release, hook release.HookEvent,
 	return shutdown()
 }
 
-type executeShutdownFunc = func() error
+type ExecuteShutdownFunc = func() error
 
 func shutdownNoOp() error {
 	return nil
 }
 
 // execHookWithDelayedShutdown executes all of the hooks for the given hook event and returns a shutdownHook function to trigger deletions after doing other things like e.g. retrieving logs.
-func (cfg *Configuration) execHookWithDelayedShutdown(rl *release.Release, hook release.HookEvent, waitStrategy kube.WaitStrategy, timeout time.Duration, serverSideApply bool) (executeShutdownFunc, error) {
+func (cfg *Configuration) execHookWithDelayedShutdown(rl *release.Release, hook release.HookEvent, waitStrategy kube.WaitStrategy, timeout time.Duration, serverSideApply bool) (ExecuteShutdownFunc, error) {
 	executingHooks := []*release.Hook{}
 
 	for _, h := range rl.Hooks {

pkg/action/release_testing.go

@@ -57,7 +57,7 @@ func NewReleaseTesting(cfg *Configuration) *ReleaseTesting {
 }
 
 // Run executes 'helm test' against the given release.
-func (r *ReleaseTesting) Run(name string) (ri.Releaser, executeShutdownFunc, error) {
+func (r *ReleaseTesting) Run(name string) (ri.Releaser, ExecuteShutdownFunc, error) {
 	if err := r.cfg.KubeClient.IsReachable(); err != nil {
 		return nil, shutdownNoOp, err
 	}