This moves the following functions meant for use from tests into a
testutil subpackage.
func DoTestCase(m Markdown, testCase MarkdownTestCase, t TestingT)
func DoTestCaseFile(m Markdown, filename string, t TestingT)
func DoTestCases(m goldmark.Markdown, cases []MarkdownTestCase, t TestingT)
This will help keep the top-level goldmark package clean and limited to
core functionality.
(Note that tests in the top-level goldmark package that make use of
these functions must now use the package name `goldmark_test` so that
they're considered separate from the main `goldmark` package, otherwise
you'll see an import cycle: goldmark imports testutil imports goldmark.)
