Fix test in go1.12

go1.12 changes the error we get when an unsupported tls version is
required and so we need to fix the test to accomodate both pre go1.12
and post go1.12

This is done through go/build instead of through files with `// +build`
as the change is very local and this looks better and is more readable
in this particular case

js/runner_test.go

@@ -26,6 +26,7 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"fmt"
+	"go/build"
 	"io/ioutil"
 	stdlog "log"
 	"net"
@@ -824,6 +825,13 @@ func TestVUIntegrationHosts(t *testing.T) {
 }
 
 func TestVUIntegrationTLSConfig(t *testing.T) {
+	var unsupportedVersionErrorMsg = "remote error: tls: handshake failure"
+	for _, tag := range build.Default.ReleaseTags {
+		if tag == "go1.12" {
+			unsupportedVersionErrorMsg = "tls: no supported versions satisfy MinVersion and MaxVersion"
+			break
+		}
+	}
 	testdata := map[string]struct {
 		opts   lib.Options
 		errMsg string
@@ -850,7 +858,7 @@ func TestVUIntegrationTLSConfig(t *testing.T) {
 		},
 		"UnsupportedVersion": {
 			lib.Options{TLSVersion: &lib.TLSVersions{Min: tls.VersionSSL30, Max: tls.VersionSSL30}},
-			"GoError: Get https://sha256.badssl.com/: remote error: tls: handshake failure",
+			"GoError: Get https://sha256.badssl.com/: " + unsupportedVersionErrorMsg,
 		},
 	}
 	for name, data := range testdata {