Add fork of Go 1.15-dev's crypto/x509

Snapshotted from Go commit 619c7a48a38b28b521591b490fd14ccb7ea5e821
(https://go-review.googlesource.com/c/go/+/229762,
"crypto/x509: add x509omitbundledroots build tag to not embed roots")

With 975c01342a25899962969833d8b2873dc8856a4f
(https://go-review.googlesource.com/c/go/+/220721) removed, because it
depends on other stuff in Go std that doesn't yet exist in a Go
release.

Also, add a subset fork of Go's internal/testenv, for use by x509's tests.

tempfork/x509/x509_test_import.go

@@ -0,0 +1,56 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build ignore
+
+// This file is run by the x509 tests to ensure that a program with minimal
+// imports can sign certificates without errors resulting from missing hash
+// functions.
+package main
+
+import (
+	"crypto/rand"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"encoding/pem"
+	"math/big"
+	"strings"
+	"time"
+)
+
+func main() {
+	block, _ := pem.Decode([]byte(pemPrivateKey))
+	rsaPriv, err := x509.ParsePKCS1PrivateKey(block.Bytes)
+	if err != nil {
+		panic("Failed to parse private key: " + err.Error())
+	}
+
+	template := x509.Certificate{
+		SerialNumber: big.NewInt(1),
+		Subject: pkix.Name{
+			CommonName:   "test",
+			Organization: []string{"Σ Acme Co"},
+		},
+		NotBefore: time.Unix(1000, 0),
+		NotAfter:  time.Unix(100000, 0),
+		KeyUsage:  x509.KeyUsageCertSign,
+	}
+
+	if _, err = x509.CreateCertificate(rand.Reader, &template, &template, &rsaPriv.PublicKey, rsaPriv); err != nil {
+		panic("failed to create certificate with basic imports: " + err.Error())
+	}
+}
+
+var pemPrivateKey = testingKey(`-----BEGIN RSA TESTING KEY-----
+MIIBOgIBAAJBALKZD0nEffqM1ACuak0bijtqE2QrI/KLADv7l3kK3ppMyCuLKoF0
+fd7Ai2KW5ToIwzFofvJcS/STa6HA5gQenRUCAwEAAQJBAIq9amn00aS0h/CrjXqu
+/ThglAXJmZhOMPVn4eiu7/ROixi9sex436MaVeMqSNf7Ex9a8fRNfWss7Sqd9eWu
+RTUCIQDasvGASLqmjeffBNLTXV2A5g4t+kLVCpsEIZAycV5GswIhANEPLmax0ME/
+EO+ZJ79TJKN5yiGBRsv5yvx5UiHxajEXAiAhAol5N4EUyq6I9w1rYdhPMGpLfk7A
+IU2snfRJ6Nq2CQIgFrPsWRCkV+gOYcajD17rEqmuLrdIRexpg8N1DOSXoJ8CIGlS
+tAboUGBxTDq3ZroNism3DaMIbKPyYrAqhKov1h5V
+-----END RSA TESTING KEY-----
+`)
+
+func testingKey(s string) string { return strings.ReplaceAll(s, "TESTING KEY", "PRIVATE KEY") }