libredirect: Add a small test

This is just a sanity check on whether the library correctly wraps the
syscalls and it's using the "true" executable for posix_spawn() and
execv().

The installCheckPhase is not executed if we are cross-compiling, so this
shouldn't break cross-compilation.

One thing I'm not actually sure is whether ${coreutils}/bin/true is
universally available on all the platforms, nor whether all the
functions we use in the test are available, but we can still fix that
after we've found out about that.

Signed-off-by: aszlig <aszlig@nix.build>