tests.overriding: tests-fetchgit: use generated non-zero hashes

Don't use zero hashes where the hash is supposed to be non-zero.

Don't depend on nix.src.hash as nix.src might be wrapped to accept patches.

pkgs/test/overriding.nix

@@ -148,11 +148,41 @@ let
       };
     };
 
+  /**
+    Take two positional arguments `fakeHash` and `partialHash`,
+    and return a modified version of fakeHash whose hash body is partially substituted by `partialHash` from the beginning,
+    used to assert a specific fake hash variant is used by an overridden FOD.
+
+    # Inputs
+
+    `fakeHash`
+
+    : The specified zero fake hash
+
+    `partialHash`
+
+    : A trimmed non-zero hash body, to substitute the beginning of the zero hash body.
+  */
+  genNonzeroFakeHash =
+    fakeHash:
+    let
+      isSRIHash = lib.hasInfix "-" fakeHash;
+      defaultHashAlgo = lib.optionalString isSRIHash lib.head (lib.splitString "-" lib.fakeHash);
+      defaultHashPrefix = lib.optionalString isSRIHash (defaultHashAlgo + "-");
+      defaultHashBody = lib.removePrefix defaultHashPrefix fakeHash;
+    in
+    partialHash:
+    defaultHashPrefix
+    + partialHash
+    + (lib.substring (lib.stringLength partialHash) (lib.stringLength defaultHashBody) defaultHashBody);
+
   tests-fetchgit =
     let
+      fakeSha256-1 = genNonzeroFakeHash lib.fakeSha256 "1";
+      fakeHash-2 = genNonzeroFakeHash lib.fakeHash "B";
       src-with-sha256 = pkgs.fetchgit {
         url = "https://example.com/source.git";
-        sha256 = lib.fakeSha256;
+        sha256 = fakeSha256-1;
       };
     in
     {
@@ -164,19 +194,19 @@ let
             ;
         };
         expected = {
-          outputHash = lib.fakeSha256;
+          outputHash = fakeSha256-1;
           outputHashAlgo = "sha256";
         };
       };
       test-fetchgit-overrideAttrs-hash = {
         expr = {
-          inherit (src-with-sha256.overrideAttrs { hash = pkgs.nix.src.hash; })
+          inherit (src-with-sha256.overrideAttrs { hash = fakeHash-2; })
             outputHash
             outputHashAlgo
             ;
         };
         expected = {
-          outputHash = pkgs.nix.src.hash;
+          outputHash = fakeHash-2;
           outputHashAlgo = null;
         };
       };