Add depth to assert_covers

[yngvar-antonsson]

Now it doesn't work:

t.assert_covers(
    {a = {b = 'c', d = 'e'}, f = 'g'}, 
    {a = {b = 'c'}}
)

Because {b = 'c', d = 'e'} ~= {b = 'c'}
I propose to add depth parameter to assert_covers to assert coverage of inner tables

[Gerold103]

I faced the same issue while trying to use this assertion for vshard config tests. Please, bump the priority. This function is barely useful now except for trivial cases.

[locker]

Done unconditionally (without a parameter) in:

• Make assert_covers recursive #388