Use omit instead of skip for test-unit

[hsbt]

I got the followings with Windows

Started
E
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Error: test_getbinaryfile_command_injection(FTPTest): NoMethodError: undefined method 'skip' for an instance of FTPTest
C:/Users/hsbt/DevDrive/github.com/ruby/ruby/gems/src/net-ftp/test/net/ftp/test_ftp.rb:2273:in 'FTPTest#test_getbinaryfile_command_injection'
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E
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Error: test_gettextfile_command_injection(FTPTest): NoMethodError: undefined method 'skip' for an instance of FTPTest
C:/Users/hsbt/DevDrive/github.com/ruby/ruby/gems/src/net-ftp/test/net/ftp/test_ftp.rb:2326:in 'FTPTest#test_gettextfile_command_injection'
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E
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Error: test_putbinaryfile_command_injection(FTPTest): NoMethodError: undefined method 'skip' for an instance of FTPTest
C:/Users/hsbt/DevDrive/github.com/ruby/ruby/gems/src/net-ftp/test/net/ftp/test_ftp.rb:2387:in 'FTPTest#test_putbinaryfile_command_injection'
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E
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Error: test_puttextfile_command_injection(FTPTest): NoMethodError: undefined method 'skip' for an instance of FTPTest
C:/Users/hsbt/DevDrive/github.com/ruby/ruby/gems/src/net-ftp/test/net/ftp/test_ftp.rb:2435:in 'FTPTest#test_puttextfile_command_injection'
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