🧪 Add experimental new FakeServer for tests

[nevans]

This was written for several reasons:

• To improve yields_in_test_server_thread and clean up tests using it
• To support new tests for ✨ Cache server capabilities and add #capable?(name) #156
• To speed up adding test coverage for commands, both new and old
• Because I kept messing up OpenSSL::SSL::SSLSocket#accept, even
  though I copied and pasted from imaps_test and starttls_test. 😳
• To support testing of other libraries and applications that use Net::IMAP.

I will (probably) eventually update most of the tests in test_imap.rb to use FakeServer. The version here is able to support the capabilities tests and several more existing tests. To quote the rdoc:

NOTE: API is experimental and may change without deprecation or warning.

FakeServer is simple fake IMAP server that is used for testing Net::IMAP. It contains simple implementations of many IMAP commands and allows customization of server responses. This allow tests to assume a more-or-less "normal" IMAP server implementation, so as to focus on what's important for what's being tested without needing to fuss over the details of a TCPServer script.

Although the API is not (yet) stable, Net::IMAP::FakeServer is also intended to be useful for testing libraries and applications which themselves use Net::IMAP.

Limitations

FakeServer cannot be a complete replacement for exploratory testing or integration testing with actual IMAP servers. Simple default behaviors will be provided for many commands, and tests may simulate specific server responses by assigning handlers (using #on).

And FakeServer is significantly more complex than simply creating a socket IO script in a separate thread. This complexity may obscure the focus of some tests or make it more difficult to debug them. Use with discretion.

Currently, the server will shutdown after a single connection has been accepted and closed. This may change in the future, but only if tests can be simplified or made significantly faster by allowing multiple connections to the same TCPServer.

The following tests in test_imap.rb have been updated to use FakeServer:

• test_clear_responses
• test_close
• test_enable
• test_responses
• test_uid_expunge
• test_uidplus_responses
• test_unselect

All of the tests that used yields_in_test_server_thread were updated, so that method was deleted too. It has effectively been replaced by with_fake_server.

[nevans]

@hsbt, @shugo I wanted to check in and make sure you're okay with this. Although I tried to keep it simple, readable, and relatively easy for someone new to use, it's significantly more complex than the simple socket.gets/socket.print scripts we were using before.

This PR added over 500 lines of code in order to save only ~50 lines of test code. I could've saved a few more lines of code, but I did add new assertions and split some test methods into multiple tests. Nevertheless, it's still an order of magnitude difference! As I continue to convert most of the existing tests and add new ones, that savings should grow.

I'm not fully satisfied with the internal structure or the API... but since I'm already using for a couple of future PRs, I wanted to add it in its current form, rather than delay until I was satisified with it... that day may never come!

My hope is that this allows the tests to focus on what matters, and thus be more approachable for us and for newcomers to read, debug, and write. But I can only answer that question for myself. After I've used it in more places and the API stabilizes I would like to either move it into lib/net/imap/test/fake_server.rb or its own gem, so it can be more easily used by other libraries and applications. What do you think?