all: sprinkle t.Parallel on some slow tests

I used the slowtests.go tool as described in
https://golang.org/cl/32684 on packages that stood out.

go test -short std drops from ~56 to ~52 seconds.

This isn't a huge win, but it was mostly an exercise.

Updates #17751

Change-Id: I9f3402e36a038d71e662d06ce2c1d52f6c4b674d
Reviewed-on: https://go-review.googlesource.com/32751
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

tls_test.go

@@ -99,6 +99,7 @@ var keyPairTests = []struct {
 }
 
 func TestX509KeyPair(t *testing.T) {
+	t.Parallel()
 	var pem []byte
 	for _, test := range keyPairTests {
 		pem = []byte(test.cert + test.key)