tests: Add some regression tests for copy_file_range()

These cover a few bugs that have cropped up, including the ones fixed by
commits 4046ad6bb0e and 2319ca6a0181.

PR: 276045
Reviewed by: rmacklem
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D51856

(cherry picked from commit 3af6f55735cef7df72ca3f4ecf2b0027abb5fcb8)