lib/shell/command-processor.rb (Shell#[]): prevent unknown command

lib/shell/command-processor.rb (Shell#[]): prevent unknown command

`FileTest.send(command, ...)` allows to call not only FileTest-related
methods but also any method that belongs to Kernel, Object, etc.
patched by <mame@ruby-lang.org>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@67814 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
[Salvatore Bonaccorso: Backport to 2.3.3:
 - Context changes in test/shell/test_command_processor.rb
]

Gbp-Pq: Name lib-shell-command-processor.rb-Shell-prevent-unknown.patch