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.
This is a Unix-specific function which adds the ability to register a closure to
run pre-exec to configure the child process as required (note that these
closures are run post-fork).
cc #31398
