avoid using Keyword.pop/2 in ExUnit.Diff.struct_module/2

[tudborg]

In ExUnit.Diff.struct_module/2 we call Keyword.pop(kw, :__struct__)
but we don't actually know if kw is a Keyword here.

I stumbled on this while looking at what @josevalim said in #15470 (comment)
I wanted to see what actually broke if we guarded Keyword functions for keyword lists.

Only few tests blew up, and only in ExUnit, examples:

• elixir/lib/ex_unit/test/ex_unit/diff_test.exs

  Lines 527 to 531 in 1d59997

  	refute_diff(
  	%{1 => :a, 2 => :b} = %{1 => :a, 12 => :b},
  	"%{1 => :a, -2 => :b-}",
  	"%{1 => :a, 12 => :b}"
  	)

• elixir/lib/ex_unit/test/ex_unit/diff_test.exs

  Lines 563 to 570 in 1d59997

  	assert_diff(
  	%{
  	"user" => ^user,
  	"subtitle" => "foo"
  	} = notification,
  	[],
  	%{{:user, nil} => user}
  	)

ExUnit.Diff.struct_module/2 is being called for both structs and maps.
(since bbf54bc, before this point we delegated to diff_map earlier)

diff_quoted_struct uses the return of struct_module to determine when to call diff_quoted_struct or diff_map, so at this point, we don't actually know if kw represents a Keyword or any other map pairs.

This fix seemed straight forward, just don't use Keyword.pop/2 to get the :__struct__ value out of kw.
I've used Enum.split_with instead, to make sure that all occurrences of :__struct__ is removed (just like Keyword.pop/2 would have)

I considered using :lists.keytake instead of Enum.split_with, but that would return at the first occurrence.

[josevalim]

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