Fix #563: Stub Kernel.system and Kernel.`

Hey @docwhat,

I've used RSpec examples metadata feature to not stub `Kernel.system` and `Kernel.backtick` methods when the `:no_system_stub` metadata is present (and I've added this metadata to `Guard::Setuper._debug_command_execution` specs).

However, there's now an error in `Guard::Setuper._debug_command_execution`...