Add spec on Process.spawn with non-existing command.
Process.spawn with non-existing command sets $? accordingly.

Errors gotten from the pipe beetween the current process and the
forked one are now matched against Errno::NOENT.
A positive match means the launched command doesn't exist in
current PATH, thus $?'s pid and exit code must be set to the dead
process' and 127 respectively.
Merge pull request #2778 from razielgn/process-spawn-exitcode

Process.spawn with non-existing command should set $? accordingly.