Use $Test::Builder::Level in TAP test functions

Use $Test::Builder::Level in TAP test functions

In TAP test functions, that is, those that produce test results, locally
increment $Test::Builder::Level.  This has the effect that test failures
are reported at the callers location rather than somewhere in the test
support libraries.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>