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/github.com/pmezard/go-difflib/README.md

@@ -1,50 +0,0 @@
-go-difflib
-==========
-
-[![Build Status](https://travis-ci.org/pmezard/go-difflib.png?branch=master)](https://travis-ci.org/pmezard/go-difflib)
-[![GoDoc](https://godoc.org/github.com/pmezard/go-difflib/difflib?status.svg)](https://godoc.org/github.com/pmezard/go-difflib/difflib)
-
-Go-difflib is a partial port of python 3 difflib package. Its main goal
-was to make unified and context diff available in pure Go, mostly for
-testing purposes.
-
-The following class and functions (and related tests) have be ported:
-
-* `SequenceMatcher`
-* `unified_diff()`
-* `context_diff()`
-
-## Installation
-
-```bash
-$ go get github.com/pmezard/go-difflib/difflib
-```
-
-### Quick Start
-
-Diffs are configured with Unified (or ContextDiff) structures, and can
-be output to an io.Writer or returned as a string.
-
-```Go
-diff := UnifiedDiff{
-    A:        difflib.SplitLines("foo\nbar\n"),
-    B:        difflib.SplitLines("foo\nbaz\n"),
-    FromFile: "Original",
-    ToFile:   "Current",
-    Context:  3,
-}
-text, _ := GetUnifiedDiffString(diff)
-fmt.Printf(text)
-```
-
-would output:
-
-```
---- Original
-+++ Current
-@@ -1,3 +1,3 @@
- foo
--bar
-+baz
-```
-