fix(output): keep the console blank line out of json and junit

[Chemaclass]

🤔 Background

Related #1243

$ bashunit --output junit tests/ > report.xml
$ xmllint --noout report.xml
report.xml:2: parser error : XML declaration allowed only at the start of the document

The document's first byte is a newline. Only the parallel path was correct — and parallel is not the default — so the plain documented command was the broken one. run_tear_down_after_script prints a blank line between files, guarded against --simple, --failures-only, --no-progress and --parallel, but not against a machine --output.

--output json gained the same leading blank line, unnoticed because JSON tolerates leading whitespace: jq accepted it, and the existing test only checks that it parses.

💡 Changes

• Suppress the blank line for json and junit, naming those two formats rather than using is_machine_output_enabled — that also covers tap, where a blank line between files is valid, streams fine and is recorded in its snapshots
• Add the sequential counterpart to the JUnit guard, which only ever pinned --parallel (the path that already worked)
• Add a real well-formedness check with xmllint, since asserting the first line is not the same as asserting a parser accepts the document
• Assert the JSON document starts where it should, not merely that it parses

Human output is unchanged: the blank line still separates files. Verified separately that escaping was never the problem — <, &, ", ]]> and backslashes, tabs and unicode all round-trip correctly in both formats, and the on-disk --report-junit/--report-json writers were unaffected.