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

.gitignore

@@ -19,3 +19,4 @@ CPackSourceConfig.cmake
 patches
 *.exe
 tutorials
+.prove