"cancellable sleep" test is flaky

[ydirson]

When building 24.39.1 in Koji, we get a high rate of failure due to this test:

From https://koji.xcp-ng.org/kojifiles/work/tasks/4261/74261/build.log:

S....S.....F
┌──────────────────────────────────────────────────────────────────────────────┐
│ [FAIL]        cancellable sleep          1   full.                           │
└──────────────────────────────────────────────────────────────────────────────┘
ASSERT Sleep was shorter than expected, total duration = 9.62ms < 10ms
FAIL Sleep was shorter than expected, total duration = 9.62ms < 10ms
Raised at Alcotest_engine__Test.check_err in file "src/alcotest-engine/test.ml", line 157, characters 20-48
Called from Stdlib__Fun.protect in file "fun.ml", line 33, characters 8-15
Re-raised at Stdlib__Fun.protect in file "fun.ml", line 38, characters 6-52
Called from Stdlib__Fun.protect in file "fun.ml", line 33, characters 8-15
Re-raised at Stdlib__Fun.protect in file "fun.ml", line 38, characters 6-52
Called from Alcotest_engine__Core.Make.protect_test.(fun) in file "src/alcotest-engine/core.ml", line 181, characters 17-23
Called from Alcotest_engine__Monad.Identity.catch in file "src/alcotest-engine/monad.ml", line 24, characters 31-35
Logs saved to `~/build/BUILD/xapi-24.39.1/_build/default/ocaml/libs/xapi-stdext/lib/xapi-fd-test/test/_build/_tests/xapi_fdcaps/cancellable sleep.001.output'.
 ──────────────────────────────────────────────────────────────────────────────
Full test results in `~/build/BUILD/xapi-24.39.1/_build/default/ocaml/libs/xapi-stdext/lib/xapi-fd-test/test/_build/_tests/xapi_fdcaps'.
1 failure! in 0.115s. 10 tests run.
make: *** [test] Error 1