Unit tests for [drains], [poison] and [slow], with apply_to=opponent

[stevecotton]

Slightly different to PR #6582, which was the 1.16 version of this. The five
lines that were labelled preserving known bug are changed to test that it's
been fixed. Once #6582 is approved, I'm intending to merge this too.

Here apply_to=opponent means that the weapon special gives the opponent the
ability, the unit that should get poisoned or slowed is the unit that has the
weapon special.

There's a known bug in 1.16, that apply_to=opponent check the wrong unit to
see it it's unpoisonable, undrainable etc. It also checks the wrong unit to
see if it's already poisoned or slowed, so a battle between two units that both
have reverse-poison results in at most one being poisoned.

Most of the credit for this is Newfrenchy's, as he's already written a fix
and a WML based test. This commit uses a Lua test instead to test more
combinations of statuses.

This adds a COMMON_KEEP_A_B_UNIT_TEST macro, which is a counterpart to the
GENERIC_UNIT_TEST macro that starts the leaders next to each other, ready
to attack. The A_B is because I'm planning a multiple-side variant too.

There's no test for [petrify], as simulate_combat doesn't provide a stat for it.

This tests only 3 of the 6 abilities whose behavior changed in 650f704.
My thoughts on testing the others are:

• [firststrike]'s test is in 650f704.
• [drains], [poison] and [slow] are tested here.
• [petrify] ends combat, it's also not exposed in simulate_combat's stats.
• [plague] triggers after combat ends.