Replace gotestyourself by gotest.tools

github.com/gotestyourself/gotestyourself moved to gotest.tools with
version 2.0.0. Moving to that one, bumping it to v2.1.0.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

vendor/gotest.tools/README.md

@@ -0,0 +1,31 @@
+# gotest.tools
+
+A collection of packages to augment `testing` and support common patterns.
+
+[![GoDoc](https://godoc.org/gotest.tools?status.svg)](https://godoc.org/gotest.tools)
+[![CircleCI](https://circleci.com/gh/gotestyourself/gotestyourself/tree/master.svg?style=shield)](https://circleci.com/gh/gotestyourself/gotestyourself/tree/master)
+[![Go Reportcard](https://goreportcard.com/badge/gotest.tools)](https://goreportcard.com/report/gotest.tools)
+
+
+## Packages
+
+* [assert](http://godoc.org/gotest.tools/assert) -
+  compare values and fail the test when a comparison fails
+* [env](http://godoc.org/gotest.tools/env) -
+  test code which uses environment variables
+* [fs](http://godoc.org/gotest.tools/fs) -
+  create temporary files and compare a filesystem tree to an expected value
+* [golden](http://godoc.org/gotest.tools/golden) -
+  compare large multi-line strings against values frozen in golden files
+* [icmd](http://godoc.org/gotest.tools/icmd) -
+  execute binaries and test the output
+* [poll](http://godoc.org/gotest.tools/poll) -
+  test asynchronous code by polling until a desired state is reached
+* [skip](http://godoc.org/gotest.tools/skip) -
+  skip a test and print the source code of the condition used to skip the test
+
+## Related
+
+* [gotest.tools/gotestsum](https://github.com/gotestyourself/gotestsum) - go test runner with custom output
+* [maxbrunsfeld/counterfeiter](https://github.com/maxbrunsfeld/counterfeiter) - generate fakes for interfaces
+* [jonboulle/clockwork](https://github.com/jonboulle/clockwork) - a fake clock for testing code that uses `time`