It makes it more obvious what calls to the ktesting API will look like in real
code.

Due a missing space, _TL was a variable of type NopTL instead
of asserting that NopTL implements the TL interface.

When testing.T.Log gets called after the test has completed, it panics. There's
also a data race (kubernetes/kubernetes#110854).

Normally that should never happen because tests should ensure that all
goroutines have stopped before returning. But sometimes it is not possible to
do that. ktesting now automatically protects against that by registering a
cleanup function and redirecting all future output into klog.

Commit 3c90bf9 accidentally changed the
default for contextual logging from "enabled" to "disabled". The intention
always was and still is to make the new API do something useful by default and
only be more cautious in key Kubernetes binaries which have a feature gate.

ktesting: handle test completion

contextual logging: enable by default again