fix(runner): keep the tear_down blank line out of machine output

[Chemaclass]

🤔 Background

Related #1297

--output promises stdout is the document. A file with a tear_down_after_script wrote a blank line there first, so the XML declaration was no longer at byte 0:

$ bashunit --no-parallel --output junit tests/ | od -c | head -1
0000000  \n   <   ?   x   m   l       v   e   r   s   i   o   n   =   "
ExpatError: XML or text declaration not at start of entity

The hook did not have to fail — an ordinary tear_down_after_script() { :; } was enough, which makes this ordinary usage rather than an edge case. --parallel skipped the line entirely, so the modes also disagreed.

💡 Changes

• The blank line after tear_down_after_script now carries the same machine-format guards its sibling path already had. tap stays excluded on purpose: it streams line by line, so a blank line between files is valid there and its snapshots record it
• --output json carried the same stray byte and only parsed because JSON tolerates leading whitespace; it is gone from that stream too

🔍 How it was found

Extending the sweep from #1295 to assert the document contract rather than the console summary: every pathological fixture × {sequential, parallel} × {json, junit}, each one actually parsed. Two further unrelated cases from that table are filed separately.