Fix stringification of abnormal Process::Status on Windows [fixup #15255]

[straight-shoota]

The change in #15255 broke Process::Status#to_s and #inspect on Windows due to the redefinition of #normal_exit?. Unfortunately we were missing specs for this, so it did go unnoticed.

This patch adds specs and restablishes the previous behaviour with the small improvement of treating the exit_status value as UInt32 instead of Int32, which results in positive numbers.

# Crystal 1.14.0
Process::Status.new(LibC::STATUS_CONTROL_C_EXIT)).inspect # => "Process::Status[-1073741510]"

# master
Process::Status.new(LibC::STATUS_CONTROL_C_EXIT)).inspect # NotImplementedError: Process::Status#exit_signal

# this patch
Process::Status.new(LibC::STATUS_CONTROL_C_EXIT)).inspect # => "Process::Status[3221225786]"

There's still room for improvement, for example map the values to names and/or use base 16 numerals as is custom for error statuses on Windows), but I'll leave that for a follow-up.