tests: fix test hang on case insenitive file systems

tests: fix test hang on case insenitive file systems

* tests/split/filter.sh: Due to an invalid 'FILE = zero.in'
construct trying to initialize a FILE variable, it would
instead try to run the FILE command which is present on
macOS 10.13 with APFS.
We also remove a redundant duplicate test clause introduced
during a rebase, and simplify the piped timeout command,
to avoid requiring a subshell and associated quoting.
* THANKS.in: Add the reporter Jack Howarth.
Fixes https://bugs.gnu.org/28506