runtime/testdata/testprog: use testenv.SyscallIsNotSupported to check syscall.Unshare
syscall.Unshare is the sort of system call that may be blocked in a
container environment, and experience has shown that different
container implementations choose from a variety of different error
codes for blocked syscalls.
In particular, the patch in
https://git.alpinelinux.org/aports/tree/community/go/tests-unshare-enosys.patch
seems to suggest that the container environment used to test the Go
distribution on Alpine Linux returns ENOSYS instead of EPERM.
The existing testenv.SyscallIsNotSupported helper checks for
the kinds of error codes we have seen from containers in practice, so
let's use that here.