The return type of ktesting.NewTestContext is now a TContext. Code
which combined it WithCancel often didn't compile anymore (cannot overwrite
ktesting.TContext with context.Context). This is a good thing because all of
that code can be simplified to let ktesting handle the cancelation.
In contrast to EtcdMain, it can be called by individual tests or benchmarks and
each caller will get a fresh etcd instance. However, it uses the same
underlying code and the same port for all instances, so tests cannot run in
parallel.
