Add export lists for files which didn't have them

Two test helper files didn't have export lists (lost during the split
of tests). This patch finally reintroduces them, to hopefully make the
export list saner and with fewer changes for purely-internal changes.

Additionally, a few missing docstrings are added as well.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>