Add T.TODO and t.Todo() to drive the TODO directive

The spec [1] suggests "not ok 13 # TODO ...", but if you attempt this
with:

  t.Fail("# TODO foo")

you get an extra hyphen ("not ok 13 - # TODO foo") which is parsed as
a failed test (and not a TODO test).

I'd initially written up an alternative to Ok:

  T.TODO(test bool, description string)

but Michael pointed out that future work may add additional test
methods (e.g. EqOk) and wanted to support those as well with something
like [2]:

  t.TODO = true
  t.Ok(Foo(), "foo")
  t.EqOk(Bar(), Baz(), "bar == baz")
  t.TODO = false  // could be done via defer too

or [2]:

  t.TODO( func () {
    t.Ok(Foo(), "foo")
    t.EqOk(Bar(), Baz(), "bar == baz")
  })

This commit adds a .TODO boolean following Michael's first proposal.

It also adds a Todo() method returning a copy of the test-state (but
setting TODO), which you can use to avoid altering the original
test-state's TODO value.  This can be useful when you don't want to
bother with a defer, or your TODO tests all live in an existing branch
of a function, or you want to chain methods for a one-off TODO test:

  t.TODO().Ok(Foo(), "foo")

To keep the count synchronized between sibling test states,
nextTestNumber is now a pointer.  We don't do anything to keep Writer
synchronized, because it's already part of the public API as a
non-pointer, and Michael is ok leaving this up to the caller [3].

[1]: http://testanything.org/tap-version-13-specification.html#todo-tests
[2]: mndrix/tap-go#6 (comment)
[3]: mndrix/tap-go#6 (comment)

Author: wking

util/tap.go

@@ -31,16 +31,20 @@ import "testing/quick"
 // T is a type to encapsulate test state.  Methods on this type generate TAP
 // output.
 type T struct {
-	nextTestNumber int
+	nextTestNumber *int
+
+	// TODO toggles the TODO directive for Ok, Fail, Pass, and similar.
+	TODO bool
 
 	// Writer indicates where TAP output should be sent.  The default is os.Stdout.
 	Writer io.Writer
 }
 
 // New creates a new Tap value
 func New() *T {
+	nextTestNumber := 1
 	return &T{
-		nextTestNumber: 1,
+		nextTestNumber: &nextTestNumber,
 	}
 }
 
@@ -74,8 +78,12 @@ func (t *T) Ok(test bool, description string) {
 		ok = "not ok"
 	}
 
-	t.printf("%s %d - %s\n", ok, t.nextTestNumber, description)
-	t.nextTestNumber++
+	if t.TODO {
+		t.printf("%s %d # TODO %s\n", ok, *t.nextTestNumber, description)
+	} else {
+		t.printf("%s %d - %s\n", ok, *t.nextTestNumber, description)
+	}
+	(*t.nextTestNumber)++
 }
 
 // Fail indicates that a test has failed.  This is typically only used when the
@@ -105,7 +113,7 @@ func (t *T) Check(function interface{}, description string) {
 
 // Count returns the number of tests completed so far.
 func (t *T) Count() int {
-	return t.nextTestNumber - 1
+	return *t.nextTestNumber - 1
 }
 
 // AutoPlan generates a test plan based on the number of tests that were run.
@@ -117,11 +125,18 @@ func escapeNewlines(s string) string {
 	return strings.Replace(strings.TrimRight(s, "\n"), "\n", "\n# ", -1)
 }
 
+// Todo returns copy of the test-state with TODO set.
+func (t *T) Todo() *T {
+	newT := *t
+	newT.TODO = true
+	return &newT
+}
+
 // Skip indicates that a test has been skipped.
 func (t *T) Skip(count int, description string) {
 	for i := 0; i < count; i++ {
-		t.printf("ok %d # SKIP %s\n", t.nextTestNumber, description)
-		t.nextTestNumber++
+		t.printf("ok %d # SKIP %s\n", *t.nextTestNumber, description)
+		(*t.nextTestNumber)++
 	}
 }