Replace Minitest with Megatest

[mattbrictson]

Megatest is largely API compatible with Minitest so this is mostly a drop-in replacement. Changes of note:

• Remove rubocop-minitest gem and config; these no longer apply
• Remove mighty_test gem; it is minitest-specific
• Megatests runs tests in parallel by default; update Latest::UpdaterTest so it doesn't cross-contaminate the file system in parallel tests
• Mocha, minitest-snapshots, and webmock don't support megatest out of the box; get them working via small adapters placed in test/support/
• Megatest automatically loads test_helper.rb so remove require "test_helper" from all tests
• Enable minitest compatibility via test_config.rb so that tests without explicit assertions are allowed