test(mountfs): exercise fsopen end-to-end in mount-API scenario

The mountfs-management integration scenario covered the new mount API
syscalls fsmount/move_mount/mount/umount/pivot_root but not fsopen, the
API's entry point and a direct eventfd-kind sibling of fsmount. Add a
best-effort fsopen("tmpfs", FSOPEN_CLOEXEC) call (closing the returned
context fd on success) and assert enter_fsopen is traced.

fsopen's tracing is otherwise correct: args[1] flags captured, args[0]
fsname (a filesystem TYPE, not a path) deliberately not treated as a
pathname, returned fd registered as the 'fsopenfd:<flags>' descriptor.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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