`assert_covers` is too strict when checking arrays

[locker]

The problem is that assert_covers now treats arrays as plain scalars while it may be convenient to check that array items cover expected values:

t.assert_covers({
    {a = 1, b = 10},
    {a = 2, b = 20},
    {a = 3, b = 30},
}, {{a = 1}, {a = 2}, {a = 3}})

This used to work now but was broken by commit 95a1c3f (merged into luatest 1.3.0), which attempted to fix #405.

What we want to ensure here is that the number of items in both arrays is the same and each item covers the corresponding expected item.