tests: lint for run-away here-doc

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>