makefs: tests: Extend the timeout of ZFS file_extend test

[jlduran]

Summary

The test makefs_zfs_tests:file_extend is timing out on ci.freebsd.org.
Increase the default timeout to allow it to finish.

Reported by: Jenkins

[jlduran]

There is really no way to tell beforehand if this timeout is enough.
Reference: https://ci.freebsd.org/view/Test/job/FreeBSD-main-aarch64-test/1867/testReport/junit/usr.sbin.makefs/makefs_zfs_tests/file_extend/

[asomers]

Again, 300s -> 400s is not much of an increase. Might want to go more.