Removed mutex from minitest.rb
Removed mutex from test.rb
Removed Minitest::Reporter.synchronize
Removed Minitest::Test.synchronize
Upon loading minitest/parallel_each, record, capture_io and capture_subprocess_io are doped with synchronization code.

Paired with @phiggins.

[git-p4: depot-paths = "//src/minitest/dev/": change = 8488]
Pushed up passed? and skipped? to Runnable since they are called from Reporter. pair:@phiggins
[git-p4: depot-paths = "//src/minitest/dev/": change = 8489]