ex_unit: Add :capture_io tag

[wojtekmach]

Possible future work:

1. Allow ExUnit.start(capture_io: true) to mimic capture_log: true?

2. Add api to programmatically append to the StringIO input buffer, right now it's only possible on creation:

   {:ok, pid} = StringIO.open("3")
   "3" = IO.gets(pid, "1 + 2")

   The idea is something like this:

   {:ok, pid} = StringIO.open("")
   
   StringIO.buffer("1 + 2")
   "1 + 2" = IO.gets(pid, "> ")
   
   :eof = IO.gets(pid, "> ")
   
   StringIO.buffer("2 + 3")
   "2 + 3" = IO.gets(pid, "> ")

   And so we could replace the usage of:

   capture_io(prompt, fn -> ... end)

   too.

   Maybe instead of calling it StringIO.buffer it could be called StringIO.input_write and StringIO.input_puts to mimic IO.write and IO.puts. Not sure if we'd need .input_binwrite too.

3. @tag capture_io: :standard_error?

   I think we could do this as long as the test is running in async: false which we'd know and then we can raise otherwise. This is as opposed to capture_io(:standard_error, ...) where ExUnit does not know if it's sync or async.

   If we do this, do we want to capture both? I guess we could accept a list of devices and set it in context:

   @tag capture_io: [:stdio, :standard_error]
   test "foo", %{capture_io: [stdio, stderr]}

   Not sure if :stdio is even semantically correct, my understanding is it's not stdio per se but the group leader. So yeah, it's probably a pass, and a reason to use capture_io in this case, but I thought I'd mention this for completeness.

4. Make the StringIO available to @tag :capture_log tests in context as %{capture_log: io}. This could be considered a breaking change since today the context is usually set as %{capture_log: true} or %{capture_log: options} but I can't think of a reason to read them back in a test.