fix(test): avoid running XAPI hooks in unit tests

[edwintorok]

If you install the XAPI RPMs in your koji build environment (e.g. to build a package that depends on XAPI) then you couldn't build XAPI again anymore because its unit tests were failing. They were failing because they found some xapi hooks installed by the previous version of XAPI, whereas normally there'd be none when the unit tests are running.

Disable running XAPI hooks during unit test, even if present we are not expected to run them.

[exception] Unix.Unix_error(Unix.ENOENT, "connect", "")
            Raised at Forkhelpers.execute_command_get_output_inner.(fun) in file "ocaml/forkexecd/lib/forkhelpers.ml", line 376, characters 10-19
            Called from Xapi_stdext_pervasives__Pervasiveext.finally in file "ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml", line 24, characters 8-14
            Re-raised at Xapi_stdext_pervasives__Pervasiveext.finally in file "ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml", line 39, characters 6-15
            Called from Xapi_hooks.execute_hook.(fun) in file "ocaml/xapi/xapi_hooks.ml", line 77, characters 10-113
            Called from Stdlib__Array.iter in file "array.ml", line 95, characters 31-48
            Called from Xapi_host.destroy in file "ocaml/xapi/xapi_host.ml", line 1108, characters 2-98
            Called from Dune__exe__Test_cluster_host.test_forget in file "ocaml/tests/test_cluster_host.ml", line 192, characters 2-42
            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

[lindig]

I have undrafted this. Is this ready, @edwintorok ?

[edwintorok]

I think I didn't run into this problem again (I use separate kojienvs to avoid the problem), but the change itself is small and safe enough, so we could probably merge it.