It looks like many of these tests are already disabled on emscripten,
which also doesn't seem to support environment variables and subprocess
spawning. Just add a similar tag for CloudABI. While there, sort some of
the lists of operating systems alphabetically.
* Store the native representation directly in the `ExitStatus` structure instead
of a "parsed version" (mostly for Unix).
* On Windows, be more robust against processes exiting with the status of 259.
Unfortunately this exit code corresponds to `STILL_ACTIVE`, causing libstd to
think the process was still alive, causing an infinite loop. Instead the loop
is removed altogether and `WaitForSingleObject` is used to wait for the
process to exit.
