Fix compilation of crypto tests on macOS
On MacOS, dune build
produces this warning:
Error: Error on dynamically loaded library: /Users/thomas/git/tezos/_opam/lib/stublibs/dllocamlevercrypt.so: dlopen(/Users/thomas/git/tezos/_opam/lib/stublibs/dllocamlevercrypt.so, 10): Library not loaded: libevercrypt.so
Referenced from: /Users/thomas/git/tezos/_opam/lib/stublibs/dllocamlevercrypt.so
Reason: image not found
As hacl-star-raw
doesn't install a .so
it's safer to only compile the test in native mode.