ssh/test: set a timeout and WaitDelay on sshd subcommands

This uses a copy of testenv.Command copied from the main repo, with
light edits to allow the testenv helpers to build with Go 1.19.

The testenv helper revealed an exec.Command leak in TestCertLogin, so
we also fix that leak and simplify server cleanup using
testing.T.Cleanup.

For golang/go#60099.
Fixes golang/go#60343.

Change-Id: I7f79fcdb559498b987ee7689972ac53b83870aaf
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/496935
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>

ssh/test/multi_auth_test.go

@@ -108,7 +108,6 @@ func TestMultiAuth(t *testing.T) {
 			ctx := newMultiAuthTestCtx(t)
 
 			server := newServerForConfig(t, "MultiAuth", map[string]string{"AuthMethods": strings.Join(testCase.authMethods, ",")})
-			defer server.Shutdown()
 
 			clientConfig := clientConfig()
 			server.setTestPassword(clientConfig.User, ctx.password)