zfs: remove zfs-tests

Removes runtime dependency on gcc and reduces closure size by more than 110MB.

pkgs/os-specific/linux/zfs/default.nix

···
 
     # Fix pkgconfig.
     ln -s ../share/pkgconfig $out/lib/pkgconfig
+
+    # Remove tests because they add a runtime dependency on gcc
+    rm -rf $out/share/zfs/zfs-tests
   '';
 
   meta = {