flakey CI failures with `Expected: 'Alcotest.reject'`

https://ocaml.ci.dev/github/ocurrent/obuilder/commit/cc1d20bed098578a559b8547799f4747997f3741/variant/freebsd-4.14_opam-2.2

We have a different flaky CI failure (in addition to #186) on the build Concurrent test:

  [FAIL]        build                 2   Concurrent.
  [OK]          build                 3   Concurrent failure.
  [OK]          build                 4   Concurrent failure 2.
  [OK]          build                 5   Cancel.
  [OK]          build                 6   Cancel 2.
  [OK]          build                 7   Cancel 3.
  [OK]          build                 8   Cancel 4.
  [OK]          build                 9   Cancel 5.
  [OK]          build                10   Delete.
  [OK]          secrets               0   Simple.
  [OK]          secrets               1   No secret provided.


┌──────────────────────────────────────────────────────────────────────────────┐
│ [FAIL]        build                 2   Concurrent.                          │
└──────────────────────────────────────────────────────────────────────────────┘
test.exe: [INFO] b1: "(from base)"
test.exe: [INFO] Base image not present; importing "base"…
test.exe: [INFO] Exec "sudo" "--" "mkdir" "-m" "755" "--" "/tmp/build_3e941d_dune/mock-store-057f13/cae662172fd450bb0cd710a769079c05bfc5d8e35efa6576edc7d0377afdd4a2-tmp/rootfs"
exec: [|sudo; --; mkdir; -m; 755; --;
        /tmp/build_3e941d_dune/mock-store-057f13/cae662172fd450bb0cd710a769079c05bfc5d8e35efa6576edc7d0377afdd4a2-tmp/rootfs|]
test.exe: [INFO] Exec "docker" "create" "--" "base"
exec: [|docker; create; --; base|]
test.exe: [INFO] Exec "docker" "export" "--" "base-2"
exec: [|docker; export; --; base-2|]
test.exe: [INFO] Exec "sudo" "--" "tar" "-C" "/tmp/build_3e941d_dune/mock-store-057f13/cae662172fd450bb0cd710a769079c05bfc5d8e35efa6576edc7d0377afdd4a2-tmp/rootfs" "-xf" "-"
exec: [|sudo; --; tar; -C;
        /tmp/build_3e941d_dune/mock-store-057f13/cae662172fd450bb0cd710a769079c05bfc5d8e35efa6576edc7d0377afdd4a2-tmp/rootfs;
        -xf; -|]
test.exe: [INFO] Exec "docker" "rm" "--force" "--" "base-2"
exec: [|docker; rm; --force; --; base-2|]
docker rm --force "base-2"
test.exe: [INFO] Exec "docker" "image" "inspect" "--format" "{{range .Config.Env}}{{print . "\x00"}}{{end}}" "--" "base"
exec: [|docker; image; inspect; --format;
        {{range .Config.Env}}{{print . "\x00"}}{{end}}; --; base|]
test.exe: [INFO] b1: "---> saved as \"cae662172fd450bb0cd710a769079c05bfc5d8e35efa6576edc7d0377afdd4a2\""
test.exe: [INFO] b1: "/: (run (shell A))"
test.exe: [INFO] b1: "A\n"
test.exe: [INFO] b2: "(from base)"
test.exe: [INFO] b2: "---> using \"cae662172fd450bb0cd710a769079c05bfc5d8e35efa6576edc7d0377afdd4a2\" from cache"
test.exe: [INFO] b2: "/: (run (shell A))"
test.exe: [INFO] b2: "A\n"
test.exe: [INFO] b2: "---> saved as \"565f16d718a4939be1275726ddfdce38a7e0fa7d29c762ccf04ed5990a72b79e\""
test.exe: [INFO] b2: "/: (run (shell C))"
test.exe: [INFO] b1: "---> saved as \"565f16d718a4939be1275726ddfdce38a7e0fa7d29c762ccf04ed5990a72b79e\""
test.exe: [INFO] b1: "/: (run (shell B))"
ASSERT timeout 10.000000s
File "src/core/lwt.ml", line 1967, character 26:
FAIL timeout 10.000000s


   Expected: `Alcotest.reject'
   Received: `Alcotest.reject'


Raised at Alcotest_engine__Test.check in file "src/alcotest-engine/test.ml", line 203, characters 4-261
Called from Lwt.Sequential_composition.map.create_result_promise_and_callback_if_deferred.callback in file "src/core/lwt.ml", line 1967, characters 26-31

I'm slightly suspicious that this may be an lwt or alcotest bug... but will need to figure out more to validate.

Start by reproducing the linked FreeBSD CI failure in the build Concurrent test and inspect the timeout reported from src/core/lwt.ml. Read the related assertion handling in src/alcotest-engine/test.ml and compare the failing and successful test cases. Done means identifying whether the flake is in Lwt or Alcotest and documenting or fixing the cause so the test is reliable.