Restore the old Lwt engine after finishing

[talex5]

Note: using ~destroy:false here is essential! Otherwise Lwt segfaults.

Reported by @tmcgilchrist - does this fix it for you?

[tmcgilchrist]

Yes that fixes the issue I was seeing.

Before:

$ dune exec -- cucumber_lwt test/test_lwt_2.feature
Done: 99% (1386/1388, 2 left) (jobs: 1)I am in the given: I have a simple background
In Manip_State with no state setting foo to 12
I am in the when: I have a thing to do
I am in the then: I have done the thing
I am in the other given
In Manip_State with no state setting foo to 12
I am in the other when
I am in the other then
I am in the given: I have a simple background
In Manip_State with no state setting foo to 12
I am in the when: I have a thing to do
I am in the then: I have done the thing
14
...
14
...
14
...
Fatal error: exception Stdlib.Effect.Unhandled(Eio__core__Suspend.Suspend(_))

After:

$ dune exec -- cucumber_lwt test/test_lwt_2.feature                        
Done: 96% (1346/1388, 42 left) (jobs: 0)I am in the given: I have a simple background
In Manip_State with no state setting foo to 12
I am in the when: I have a thing to do
I am in the then: I have done the thing
I am in the other given
In Manip_State with no state setting foo to 12
I am in the other when
I am in the other then
I am in the given: I have a simple background
In Manip_State with no state setting foo to 12
I am in the when: I have a thing to do
I am in the then: I have done the thing
14
...
14
...
14
...

[talex5]

Thanks for checking!