tap: add option to switch output to TAP

Add internal flag int::output_tap to indicate we wish to only print TAP
friendly output.
tap: remove timestamp and colors from TAP output
tap: make output strings TAP firendly

This commit changes the output even for normal output mode. This is
motivated by the advantage of keeping the same code path for normal and
TAP output.
tap: add SKIP and PLAN result types
tap: add test number to output of result