go/analysis/.../unitchecker: suppress test unless go1.12

Change-Id: I1af89b9e077cf7eddeeeed72e66cd88f650492a2
Reviewed-on: https://go-review.googlesource.com/c/147197
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>

go/analysis/internal/unitchecker/unitchecker_test.go

@@ -2,8 +2,13 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build go1.12
+
 package unitchecker_test
 
+// This test depends on go1.12 features such as go vet's support for
+// GOVETTOOL, and the (*os/exec.ExitError).ExitCode method.
+
 import (
 	"flag"
 	"log"