Don't write TAP header until TAP output is available

[vweevers]

If you run non-tap tools (like dependency-check, jshint, etc) before the tap tests, their output (stderr, maybe stdout too provided that tap-parser does not wait for the header) should not interject tap output. It's esp. ugly with reporters like faucet, that swallow the header:

# TAP version 13
garbled non-tap output here..✓ first test
✓ second test
✓ etc

it should be:

non-tap output here..
✓ first test
✓ second test
✓ etc