Remove "testing" import from public interface

Currently, the DoTestCase, DoTestCaseFile, and DoTestCases functions,
are exposed as part of the public interface.

    func DoTestCase(m Markdown, testCase MarkdownTestCase, t *testing.T)
    func DoTestCaseFile(m Markdown, filename string, t *testing.T)
    func DoTestCases(m goldmark.Markdown, cases []MarkdownTestCase, t *testing.T)

Implementing these functions requires importing the `testing` package.
Importing the `testing` package [automatically registers][1] a number of
global [command line flags][2].

  [1]: https://golang.org/src/testing/testing.go#L252
  [2]: https://golang.org/cmd/go/#hdr-Testing_flags

The effect of this is that any application using the standard `flag`
package with goldmark will automatically get a number of unwanted
command line flags. This is verifiable with the following program,

    package main

    import (
    	"flag"

    	_ "github.com/yuin/goldmark"
    )

    func main() {
    	flag.Parse()
    }

To fix this, the `testing` import needs to be removed from all non-test
files. There are two ways to go about it,

- If the functions are meant for external use, you can define an
  interface with a subset of the methods of `testing.T` and switch the
  functions to consume that. This is what testing libraries like
  [gomock] and [testify] do.
- If the functions are meant for internal use, you can remove them from
  the public interface of the library and use them only from tests.

  [gomock]: https://godoc.org/github.com/golang/mock/gomock#TestReporter
  [testify]: https://godoc.org/github.com/stretchr/testify/require#TestingT

Since these functions are meant to be used by external extensions, I've
introduced a TestingT interface that is a subset of the functionality
provided by `testing.T`. It supports the standard operations: logging,
skiping, and failing tests,

testutil.go

@@ -4,14 +4,22 @@ import (
 	"bufio"
 	"bytes"
 	"fmt"
-	"github.com/yuin/goldmark/util"
 	"os"
 	"runtime/debug"
 	"strconv"
 	"strings"
-	testing "testing"
+
+	"github.com/yuin/goldmark/util"
 )
 
+// TestingT is a subset of the functionality provided by testing.T.
+type TestingT interface {
+	Logf(string, ...interface{})
+	Skipf(string, ...interface{})
+	Errorf(string, ...interface{})
+	FailNow()
+}
+
 type MarkdownTestCase struct {
 	No       int
 	Markdown string
@@ -21,7 +29,7 @@ type MarkdownTestCase struct {
 const attributeSeparator = "//- - - - - - - - -//"
 const caseSeparator = "//= = = = = = = = = = = = = = = = = = = = = = = =//"
 
-func DoTestCaseFile(m Markdown, filename string, t *testing.T) {
+func DoTestCaseFile(m Markdown, filename string, t TestingT) {
 	fp, err := os.Open(filename)
 	if err != nil {
 		panic(err)
@@ -77,13 +85,13 @@ func DoTestCaseFile(m Markdown, filename string, t *testing.T) {
 	DoTestCases(m, cases, t)
 }
 
-func DoTestCases(m Markdown, cases []MarkdownTestCase, t *testing.T) {
+func DoTestCases(m Markdown, cases []MarkdownTestCase, t TestingT) {
 	for _, testCase := range cases {
 		DoTestCase(m, testCase, t)
 	}
 }
 
-func DoTestCase(m Markdown, testCase MarkdownTestCase, t *testing.T) {
+func DoTestCase(m Markdown, testCase MarkdownTestCase, t TestingT) {
 	var ok bool
 	var out bytes.Buffer
 	defer func() {

testutil_test.go

@@ -0,0 +1,7 @@
+package goldmark
+
+import "testing"
+
+// This will fail to compile if the TestingT interface is changed in a way
+// that doesn't conform to testing.T.
+var _ TestingT = (*testing.T)(nil)