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.conf

@@ -22,7 +22,6 @@ github.com/golang/protobuf v1.1.0
 github.com/opencontainers/runtime-spec v1.0.1
 github.com/opencontainers/runc 69663f0bd4b60df09991c08812a60108003fa340
 github.com/sirupsen/logrus v1.0.0
-github.com/pmezard/go-difflib v1.0.0
 github.com/urfave/cli 7bc6a0acffa589f415f88aca16cc1de5ffd66f9c
 golang.org/x/net b3756b4b77d7b13260a0a2ec658753cf48922eac
 google.golang.org/grpc v1.12.0
@@ -40,7 +39,7 @@ google.golang.org/genproto d80a6e20e776b0b17a324d0ba1ab50a39c8e8944
 golang.org/x/text 19e51611da83d6be54ddafce4a4af510cb3e9ea4
 github.com/stevvooe/ttrpc d4528379866b0ce7e9d71f3eb96f0582fc374577
 github.com/syndtr/gocapability db04d3cc01c8b54962a58ec7e491717d06cfcc16
-github.com/gotestyourself/gotestyourself 44dbf532bbf5767611f6f2a61bded572e337010a
+gotest.tools v2.1.0
 github.com/google/go-cmp v0.1.0
 
 github.com/containerd/cri 8bcb9a95394e8d7845da1d6a994d3ac2a86d22f0