DoTestCaseFile: Don't panic

Instead of panicking, report the error in the form,

    $file: $msg

So an error might look like,

    foo.txt: line 12: invalid case No: [..]

And then kill the test with FailNow.
This is the same as calling `t.Fatalf(...)`.

testutil/testutil.go

@@ -236,7 +236,8 @@ func ParseTestCaseFile(filename string) ([]MarkdownTestCase, error) {
 func DoTestCaseFile(m goldmark.Markdown, filename string, t TestingT, no ...int) {
 	allCases, err := ParseTestCaseFile(filename)
 	if err != nil {
-		panic(err)
+		t.Errorf("%v: %v", filename, err)
+		t.FailNow()
 	}
 
 	cases := allCases[:0]