This closes a gap compared to the context package. It's useful when combined
with Ginkgo to keep something running beyond the end of the Ginkgo BeforeEach
or It node.
That WithCancel added a deferred cleanup which cancels on test termination was
unexpected. This automatic cancellation makes sense only for the initial root
TContext.
The new TContext interface combines a normal context and the testing interface,
then adds some helper methods. The context gets canceled when the test is done,
but that can also be requested earlier via Cancel.
The intended usage is to pass a single `tCtx ktesting.TContext` parameter
around in all helper functions that get called by a unit or integration test.
Logging is also more useful: Log[f] and Fatal[f] output is prefixed with
"[FATAL] ERROR: " to make it stand out more from regular log output.
If this approach turns out to be useful, it could be extended further (for
example, with a per-test timeout) and might get moved to a staging repository
to enable usage of it in other staging repositories.
To allow other implementations besides testing.T and testing.B, a custom
ktesting.TB interface gets defined with the methods expected from the
actual implementation. One such implementation can be ginkgo.GinkgoT().
