Test fd_mmap Memory Implementation

[theduke]

#3422 added a fd_mmap memory implementation that is located in the wasmer-sys-utils crate.

This implementation has active users, but currently only a commented out test. (in lib/sys-utils/tests).

The test should be re-enabled, and the memory implementation should get extensive test coverage.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

Feel free to reopen the issue if it has been closed by mistake.