fuzz: use new </> operators in fuzz_test.rs

[cakebaker]

This PR uses the new </> operators, introduced with #7315, in fuzz_test.rs. The PR also bumps parse_datetime from 0.7 to 0.8.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/test/test is no longer failing!

[sylvestre]

Great idea :)