fix: HTML report merge crash and harden result file handling

[mkzung]

Four fixes in the result-merging / report-generation path, plus the repo's first tests/test_utils.py.

1. merge_html crashed, then produced broken HTML (utils.py). It searched for a <thread> tag (not valid HTML; the tag is <thead>) and indexed [0], so find_all('thread')[0] raised IndexError and crashed the merge. Beyond the crash, the row merge appended each second-table <tr> into the first table's <tr>, nesting rows and putting a data row inside <thead>. Since the merged tables are the same client's results from two report folders (each client has a unique id="table_<client>"), the intent is to stack rows under one header. This removes the duplicate <thead> (decomposing every match, so it also handles a table with none) and appends the second table's body rows into the first table's <tbody>, producing valid, correctly-stacked HTML.
2. merge-results.py. Replaced os.system(f'cp {src} {dst}') with shutil.copy, so paths with spaces or shell metacharacters are handled correctly.
3. report_html.py tags. Fixed a malformed <pre"> tag (should be <pre>).
4. report_html.py table close order. The report emitted </table> before </tbody>; swapped them to close </tbody> then </table>.

Adds tests/test_utils.py (matching the naming of the existing suite) asserting the merged output has one header, all rows stacked in the body, and no nested <tr>, plus the no-<thead> case; a root conftest.py makes the top-level modules import under pytest. Run with python -m pytest tests/test_utils.py -v. Verified locally that the merged output renders as valid, correctly-stacked HTML.