compatibility with lwt.6

[raphael-proust]

No description provided.

[talex5]

CI lower bounds check failed with this (probably just needs a lower-bound added):

File "lib/lwt_eio.ml", line 126, characters 5-25:
type Lwt_engine.engine_id += Engine_id__Eio_lwt
     ^^^^^^^^^^^^^^^^^^^^
Error: Unbound type constructor Lwt_engine.engine_id

With Lwt 6, the tests need updating due to a change in the output ordering (assuming that's expected):

--- a/_build/default/test/test.md
+++ b/_build/default/test/.mdx/test.md.corrected
@@ -41,20 +41,20 @@ Lwt and Eio fibers don't block each other:
);;
 +eio: i = 1
 +  lwt: i = 1
-+  lwt: i = 2
 +eio: i = 2
-+  lwt: i = 3
++  lwt: i = 2

[raphael-proust]

will make the changes (and check the tests), thanks

[raphael-proust]

made the changes

i don't know why the tests change, i'll try to read the lwt_eio internal to understand what's the synchronisation mechanism there