unit testing of SetLogger

[pohly]

/kind feature

Describe the solution you'd like

#280 broke stack unwinding when using a logr.Logger as logging implementation. This was caught when trying out the modified klog before a release with these tests:

https://github.com/kubernetes/kubernetes/blob/master/staging/src/k8s.io/component-base/logs/json/klog_test.go

Here's an idea how we can prevent such regressions sooner in the future and provide more consistent testing:

• split out the formatting code in klog.go into a stand-alone Logger that has its own independent state - already partly done for the testing logger that I worked on a while ago
• copy the klog_test.go into a "klog/tests" package and modify them so that other logger implementations can reuse them by overriding the expected output checks
• extend the tests to cover various corner cases that Logger's have to support:
  • KObj handling
  • panics in functions provided by values (String, Error, MarshalLog)
  • different value types
  • duplicate key/value pairs
  • etc. - unit tests in the various Logger implementations should provide plenty of ideas