The numbered tests are from https://wiki.php.net/rfc/php7_foreach, in the
order that they appear in that document.

Tests where we (currently) don't follow php7 behavior are .disabled, and
contain comments that explain why.

2 of the tests were specified in a non-behavior-altering form, and then their
expected behavior was modified by an "additional behavior change" that had a
separate vote. These tests have a and b versions; it is not possible for both
versions to pass, but the existence of both with one .disabled documents which
behavior we follow.

Additional tests related to foreach by reference behavior (but not from the
above RFC) have the prefix extra_
