Change-ID: go~master~Ia979f971b9bf581b230d7d240ad4068e594eefe0 Commit: 48b8a5302be1746586011a115ed91f030372343f (status)
   Remain: 0, fails: [freebsd-amd64-12_0 linux-amd64 linux-386 windows-amd64-2016 windows-386-2008 linux-amd64-race openbsd-amd64-68 android-amd64-emu js-wasm]
  android-amd64-emu: running=false
  freebsd-amd64-12_0: running=false
  js-wasm: running=false
  linux-386: running=false
  linux-amd64: running=false
  linux-amd64-race: running=false
  misc-compile-darwin: running=false
  misc-compile-darwinarm64: running=false
  misc-compile-freebsd: running=false
  misc-compile-linuxarm: running=false
  misc-compile-mips: running=false
  misc-compile-netbsd: running=false
  misc-compile-openbsd: running=false
  misc-compile-other: running=false
  misc-compile-plan9: running=false
  misc-compile-ppc: running=false
  misc-compile-solaris: running=false
  openbsd-amd64-68: running=false
  windows-386-2008: running=false
  windows-amd64-2016: running=false
  linux-amd64: running=false

testing: add diffing to clearly discern Example test failure output

Modifies Example test failure output to instead show a diff between
gotten and expected output. This ensures that users can see exactly
where the changes occured. It uses github.com/pkg/diff, because
out of the 2 final best diffing packages I examined, this one produced
the correct output that's very familiar to most developers.

An exhibit of this new change would be:

package p
import  "fmt"

func ExampleHello() {
        fmt.Printf("Hello \nWorld\nFuwer\n")

        // Output:
        // Pello
        // xorld
        // Fewer
}

which when run with "go test", produces the following output:

Change-Id: Ia979f971b9bf581b230d7d240ad4068e594eefe0

--- FAIL: ExampleHello (0.00s)
Output mismatch:
--- got
+++ want
@@ -1,3 +0,0 @@
-Hello
-World
-Fuwer
@@ -0,0 +1,3 @@
+Pello
+xorld
+Fewer

Thanks to the efforts led by Joshua Bleecher Snyder for implementing in Go,
the Eugene. W. Myers diffing algorithm in github.com/pkg/diff that's being used.

This change does a straight up go mod, vendor and import of github.com/pkg/diff
because that code is licensed to Joshua, so can't be directly brought into the Go
tree as an internal package until we've negotiated with him, but this CL is a start
to kickstart the conversation.

Fixes #41980

Change-Id: I5e1cd3357a5ae202a902060935b99fd1a821ba59