skipper: refactor test options

[AlexanderYastrebov]

Remove unexported test options to allow using them outside of skipper package.

• refactor testOptions into Options fields
• add shutdown channels

• requires skipper: fix TestDataClients #2418

[AlexanderYastrebov]

This would be used by #2416

[szuecs]

Do not expose channels in skipper library, because it is not a good api for library users and I don't see why we would need it.

[szuecs]

We discussed that you can hopefully use export_test.go to expose an API to pass channels, if routesrv does it similar we can not use this to start both and then it's fine to pass it via Options.

[AlexanderYastrebov]

Do not expose channels in skipper library, because it is not a good api for library users and I don't see why we would need it.

Currently there is no way to run skipper within the same process (e.g. from the test) and properly shutdown it because it only shuts down on SIGTERM.

This change does not expose any internal channels but allows user to specify optional channels to trigger and signal shutdown.

[AlexanderYastrebov]

if routesrv does it similar

routesrv also does not support programmatic shutdown atm

[AlexanderYastrebov]

We need to think more about programmatic shutdown for both skipper and routesrv.
Besides adding channels to options we may add exported RunWithShutdown functions.

Closing this for now in favor of #2419