Add unit tests for package safe

[gottwald]

This PR adds tests to package safe.

In some cases the package is a bit awkward to test.
I also noticed something strange in the code here: https://github.com/containous/traefik/blob/e2fdc27d642c9436c9b925e956290e7df95c43bf/safe/routine.go#L110-L117

Pool.Start() starts the Pool.routines, but the only way to add a routine (not routineCtx) from outside of the package is by calling Pool.Go(goroutine), which already starts the goroutine.

The code was added with this commit when routineCtx got introduced: bea5ad3#diff-f7272237e30bd2c1dc9ccb7f7b2ce583R89
Maybe an issue should be opened to find out if Pool.routines started by Pool.Start() are used at all.

Due to the tests I also found a bug in the Pool.Start() implementation, where a copy of a struct was modified instead of the struct itself.

[ldez]

Awesome job! The tests are gold ❤️

[timoreimann]

LGTM 👍

[emilevauge]

@gottwald

I also noticed something strange in the code here: https://github.com/containous/traefik/blob/e2fdc27d642c9436c9b925e956290e7df95c43bf/safe/routine.go#L110-L117

Pool.Start() starts the Pool.routines, but the only way to add a routine (not routineCtx) from outside of the package is by calling Pool.Go(goroutine), which already starts the goroutine.

The reason is to relaunch all the routines after a call to Stop. This is used in cluster context where we have to launch some specific go routines in a master instance.

[emilevauge]

Awesome work @gottwald
LGTM