Improve TAP compliance in test suite

This allows tools like `prove` to operate on our test-suite, allowing us
to do things like run recently-failing tests first, randomising test
order, and fancier reporting options.

Changes made:

- TAP output always goes to STDOUT, never to STDERR
- Skipped tests signaled as `ok 1 # skip` as per standard
- Expected failures signaled as `not ok 1 # TODO` as per standard
- `.prove` added to .gitignore

test/test.cpp

@@ -452,11 +452,12 @@ void UnitTest::skip (const std::string& text)
 {
   ++_counter;
   ++_skipped;
-  std::cout << yellow ("skip")
+  std::cout << yellow ("ok")
             << " "
             << _counter
             << " - "
             << text
+            << " # skip"
             << "\n";
 }