cmd/gorename: log 'go build' output on failure

For golang/go#50043

Change-Id: I90d1e85c59d6f9c9ad30edf0180cb876c2564534
Reviewed-on: https://go-review.googlesource.com/c/tools/+/370254
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

cmd/gorename/gorename_test.go

diff --git a/cmd/gorename/gorename_test.go b/cmd/gorename/gorename_test.go
index 2928051..30b8796 100644
--- a/cmd/gorename/gorename_test.go
+++ b/cmd/gorename/gorename_test.go
@@ -331,8 +331,8 @@
 		bin += ".exe"
 	}
 	cmd := exec.Command("go", "build", "-o", bin)
-	if err := cmd.Run(); err != nil {
-		t.Fatalf("Building gorename: %v", err)
+	if out, err := cmd.CombinedOutput(); err != nil {
+		t.Fatalf("Building gorename: %v\n%s", err, out)
 	}
 	return tmp, bin, func() { os.RemoveAll(tmp) }
 }