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I'm curious: is it possible to query the filesystem to check if a copied is copied using CoW? I guess that it's possible, it will be non portable. So I guess that it's better to avoid checking that in unit tests.
vstinner@apu$ dd if=/dev/urandom of=urandom bs=1k count=1k
1024+0 records in
1024+0 records out
1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.0309671 s, 33.9 MB/s
vstinner@apu$ cp --reflink urandom urandom2
'urandom' -> 'urandom2'
vstinner@apu$ stat urandom
 File: urandom
 Size: 1048576 	Blocks: 2048 IO Block: 4096 regular file
Device: 31h/49d	Inode: 16265363 Links: 1
Access: (0664/-rw-rw-r--) Uid: ( 1000/vstinner) Gid: ( 1000/vstinner)
Context: unconfined_u:object_r:user_home_t:s0
Access: 2019年06月05日 13:56:21.381196972 +0200
Modify: 2019年06月05日 13:56:21.412197007 +0200
Change: 2019年06月05日 13:56:21.412197007 +0200
 Birth: 2019年06月05日 13:56:21.381196972 +0200
vstinner@apu$ stat urandom2
 File: urandom2
 Size: 1048576 	Blocks: 2048 IO Block: 4096 regular file
Device: 31h/49d	Inode: 16265364 Links: 1
Access: (0664/-rw-rw-r--) Uid: ( 1000/vstinner) Gid: ( 1000/vstinner)
Context: unconfined_u:object_r:user_home_t:s0
Access: 2019年06月05日 13:56:24.487200453 +0200
Modify: 2019年06月05日 13:56:24.496200463 +0200
Change: 2019年06月05日 13:56:24.496200463 +0200
 Birth: 2019年06月05日 13:56:24.487200453 +0200
Using stat command line tool, I don't see anything obvious saying that the two files share the same data on disk.