Helps issue #1434

Merging tests/child-test.c into glib/tests/spawn-multithreaded.c

See merge request GNOME/glib!2390

This warning warns on widening casts from integer to pointer type,
because those casts can be problematic for porting to new pointer
sizes. The code in question didn't do any bad things here so an
intermediate cast to size_t was added to silence the warning

This test was exploiting unspecified behavior w.r.t. the address of string
literals, It expected them to be pooled (the same literal has the same
address, at least within a TU), but MSVC does not pool by default,
leading to a failure.

Helps issue #1434

Reporting these as dup2() failures is bogus.

Although unlikely, these functions can fail, e.g. if we run out of file
descriptors. Check for errors to improve robustness. This is especially
important now that I changed our use of dupfd_cloexec() to avoid
returning fds smaller than the largest fd in target_fds. An application
that attempts to remap to the highest-allowed fd value deserves at least
some sort of attempt at error reporting, not silent failure.

This means that GSubprocess will (sometimes) be able to use the
optimized posix_spawn codepath instead of having to fall back to
fork/exec.