testing: rename testContext to testState

Following up to CL 603959, update internals of testing package to
reduce the confusion around "context". The changes rename
testContext/benchContext/fuzzContext to testState/benchState/fuzzState.

Change-Id: Ib8855dab456d41ab343488fcf5fefff2431f7b72
Reviewed-on: https://go-review.googlesource.com/c/go/+/607555


Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>