Disable Lin Bytes test under OCaml 5.2 and earlier

[jmid]

The weekly runs this morning caused all 5.2.0 runs to fail the Lin Bytes test.

This was an omission from #546:

The new improved ™️ Thread mode utilizes Gc.Memprof support to trigger context switching. 5.3 restored Memprof, which means the thread test works well there and on trunk - both tested by CI on PRs.
I tested locally on 5.2 at some point and then forgot about it again. Given the above, I think the Lin Thread-mode test of Bytes should just be disabled on 5.2 and earlier.

This PR does exactly that.

[shym]

LGTM, thank you!

[jmid]

CI summary: all 36 workflows passed

[jmid]

CI summary for merge to main: all 37 workflows passed