Stop the disk size tests failing at random on ZFS (closes #150)

unix.Sync can return before ZFS has counted a file's blocks, so a test
that expected the size on disk to be settled after it failed at random.
The test that needs the size to change now calls unix.Sync in a loop
until it does, for at most 30 seconds. All three tests read the size
before and after the scan and accept either value.

Model: opus-5-5

TODO.md

@@ -29,6 +29,12 @@
 
 # Completed Steps
 
+- the three disk size tests in `scan_test.go` no longer fail at random on ZFS,
+  where `unix.Sync` can return before a file's blocks are counted. The test that
+  needs the size to change waits for it, for at most 30 seconds. Each test reads
+  the size before and after its scan and accepts either value (2026-11-02,
+  https://git.eeqj.de/sneak/sfdupes/issues/150)
+
 - `README.md` rewritten from scratch, sentence by sentence, as a newcomer reads
   it. It leads with what the program is and how to use it. A reference section
   covers every setting and every subcommand. Development and testing come last.