nixos/tests/zfs: decouple makeZfsTest params from unstable vs. stable

enableUnstable was trying to do too much, it’s more obvious to remove
the indirection to set zfsPackage and push the semantics up to the
caller.

nixos/tests/zfs.nix

@@ -8,12 +8,9 @@ with import ../lib/testing-python.nix { inherit system pkgs; };
 let
 
   makeZfsTest = name:
-    { kernelPackages ? if enableUnstable
-                       then pkgs.zfsUnstable.latestCompatibleLinuxPackages
-                       else pkgs.linuxPackages
-    , enableUnstable ? false
+    { kernelPackages
     , enableSystemdStage1 ? false
-    , zfsPackage ? if enableUnstable then pkgs.zfsUnstable else pkgs.zfs
+    , zfsPackage
     , extraTest ? ""
     }:
     makeTest {
@@ -197,16 +194,22 @@ in {
   # maintainer: @raitobezarius
   series_2_1 = makeZfsTest "2.1-series" {
     zfsPackage = pkgs.zfs_2_1;
+    kernelPackages = pkgs.linuxPackages;
   };
 
-  stable = makeZfsTest "stable" { };
+  stable = makeZfsTest "stable" {
+    zfsPackage = pkgs.zfsStable;
+    kernelPackages = pkgs.linuxPackages;
+  };
 
-  unstable = makeZfsTest "unstable" {
-    enableUnstable = true;
+  unstable = makeZfsTest "unstable" rec {
+    zfsPackage = pkgs.zfsUnstable;
+    kernelPackages = zfsPackage.latestCompatibleLinuxPackages;
   };
 
-  unstableWithSystemdStage1 = makeZfsTest "unstable" {
-    enableUnstable = true;
+  unstableWithSystemdStage1 = makeZfsTest "unstable" rec {
+    zfsPackage = pkgs.zfsUnstable;
+    kernelPackages = zfsPackage.latestCompatibleLinuxPackages;
     enableSystemdStage1 = true;
   };