Omit flaky test with macOS platform

[hsbt]

For #287

OpenSSL::SSL::SSLError is raised at 13/69 times in my macOS box. I always retried all of test suite of ruby/ruby everyday.

I know we should resolve this with root causes. But I have not enough time to investigate this.

And I got the following timeout issue with 4 times.

#<Thread:0x00000003236f3850 /Users/hsbt/Documents/github.com/ruby/ruby/gems/src/net-imap/test/net/imap/fake_server/test_helper.rb:10 run> terminated with exception (report_on_exception is true):
/Users/hsbt/Documents/github.com/ruby/ruby/lib/timeout.rb:40:in 'Timeout::Error.handle_timeout': execution expired (Timeout::Error)

I extend that timeout value to handle that.

I didn't get test failed running with ten times after applying above two workarounds.

[nevans]

@hsbt This is reasonable.