ssh/agent: clear the environment when starting ssh-agent in client_test

Certain environment variables can influence the behavior of ssh-agent,
causing the test to fail. Avoid that influence by using a consistent
environment.

This fixes a locally-observed test failure for me.

Change-Id: I0f5e8d643199519f88e80825335ee8e6eb08e3af
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/207901
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>

ssh/agent/client_test.go

@@ -14,6 +14,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"strconv"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -35,17 +36,19 @@ func startOpenSSHAgent(t *testing.T) (client ExtendedAgent, socket string, clean
 	}
 
 	cmd := exec.Command(bin, "-s")
+	cmd.Env = []string{} // Do not let the user's environment influence ssh-agent behavior.
+	cmd.Stderr = new(bytes.Buffer)
 	out, err := cmd.Output()
 	if err != nil {
-		t.Fatalf("cmd.Output: %v", err)
+		t.Fatalf("%s failed: %v\n%s", strings.Join(cmd.Args, " "), err, cmd.Stderr)
 	}
 
-	/* Output looks like:
+	// Output looks like:
+	//
+	//	SSH_AUTH_SOCK=/tmp/ssh-P65gpcqArqvH/agent.15541; export SSH_AUTH_SOCK;
+	//	SSH_AGENT_PID=15542; export SSH_AGENT_PID;
+	//	echo Agent pid 15542;
 
-		   SSH_AUTH_SOCK=/tmp/ssh-P65gpcqArqvH/agent.15541; export SSH_AUTH_SOCK;
-	           SSH_AGENT_PID=15542; export SSH_AGENT_PID;
-	           echo Agent pid 15542;
-	*/
 	fields := bytes.Split(out, []byte(";"))
 	line := bytes.SplitN(fields[0], []byte("="), 2)
 	line[0] = bytes.TrimLeft(line[0], "\n")