Eio_mock.Backend: preserve backtraces from main

[talex5]

Tested with:

open Eio.Std

let test () : unit =
  failwith "here"

let () =
  Printexc.record_backtrace true;
  Eio_mock.Backend.run @@ fun () ->
  test ();
  traceln "done"

Before:

Fatal error: exception Failure("here") 
Raised at Eio_mock__Backend.run.fork.(fun) in file "lib_eio/mock/backend.ml", line 29, characters 55-63
Called from Eio_mock__Backend.run in file "lib_eio/mock/backend.ml", line 57, characters 23-75
Called from Dune__exe__Test in file "example/test.ml", line 8, characters 2-63

After:

Fatal error: exception Failure("here") 
Raised at Stdlib.failwith in file "stdlib.ml", line 29, characters 17-33
Called from Dune__exe__Test.test in file "example/test.ml" (inlined), line 4, characters 2-17
Called from Dune__exe__Test.(fun) in file "example/test.ml", line 9, characters 2-9
Called from Eio_mock__Backend.run.(fun) in file "lib_eio/mock/backend.ml", line 61, characters 65-74
Re-raised at Eio_mock__Backend.run.fork.(fun) in file "lib_eio/mock/backend.ml", line 32, characters 12-47
Called from Eio_mock__Backend.run in file "lib_eio/mock/backend.ml", line 61, characters 23-75
Called from Dune__exe__Test in file "example/test.ml", line 8, characters 2-63