tests: lint for run-away here-doc

We found a few run-away here documents that are started with an end-of-here-doc marker that is incorrectly spelled, e.g. git some command >actual && cat <<EOF >expect ... EOF && test_cmp expect actual which ends up slurping the entire remainder of the script as if it were the data. Often the command that gets misused like this exits without failure (e.g. "cat" in the above example), which makes the command appear to work, without ever executing the remainder of the test. Piggy-back on the test that catches &&-chain breakage to detect this case as well. Signed-off-by: Junio C Hamano <gitster@pobox.com>