Fix data race in mockbeat (#5037)

[adriansr]

Mockbeat expects Beater.Stop to be called after Beater.Run has
initialized some shared state. This is a race as Stop can be called at
any moment, even before Run is ever called.

This caused race warnings and segfaults when running unit tests.

Closes #5037

[urso]

This is a general problem all beats currently have. The Stop method might need to cleanup/close state/variables that are only created during the run method. Reason Stop closes the client is, if the client is blocking the beat will never stop.

+100 If this PR really solves some issues when using -race.

But longterm I'd like to have some support to signal shutdown in the run method, potentially removing the Stop method. e.g. think context.Context. Or something more tailored like:

type Signal struct { ... }

func (s *Signal) C() <-chan struct{} { ... }

func (s *Signal) CloseOnSig(c io.Closer) { ... }
func (s *Signal) CBOnSig(fn func()) { ... }

[tsg]

LGTM, nice work! The test failures are because of the sha512 files, they should go away if you rebase to master.

[andrewkroh]

LGTM