unix: fix MremapPtr test failing on NetBSD

[ncruces]

NetBSD apparently doesn't allow remapping into used address space.
This means that the test that uses mremap to move a mmapped page
into a new address should first mmap (to reserve)
then munmap (to free) the destination.

Fixes golang/go#68180

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This PR is being closed because golang.org/cl/594756 has been merged.