Try #1732:

Author: iohk-bors[bot]

builder/comp-builder.nix

@@ -454,11 +454,13 @@ let
         target-pkg-and-db = "${ghc.targetPrefix}ghc-pkg -v0 --package-db $out/package.conf.d";
       in ''
       runHook preInstall
-      $SETUP_HS copy ${lib.concatStringsSep " " (
-        setupInstallFlags
-        ++ lib.optional configureAllComponents
-              (haskellLib.componentTarget componentId)
-      )}
+      ${ # `Setup copy` does not install tests and benchmarks.
+        lib.optionalString (!haskellLib.isTest componentId && !haskellLib.isBenchmark componentId) ''
+          $SETUP_HS copy ${lib.concatStringsSep " " (
+            setupInstallFlags
+            ++ lib.optional configureAllComponents
+                  (haskellLib.componentTarget componentId)
+          )}''}
       ${lib.optionalString (haskellLib.isLibrary componentId) ''
         $SETUP_HS register --gen-pkg-config=${name}.conf
         ${ghc.targetPrefix}ghc-pkg -v0 init $out/package.conf.d

test/default.nix

@@ -215,6 +215,7 @@ let
     exe-lib-dlls = callTest ./exe-lib-dlls { inherit util; };
     ca-derivations = callTest ./ca-derivations { inherit CADerivationsEnabled; };
     ca-derivations-include = callTest ./ca-derivations-include { inherit CADerivationsEnabled; };
+    test-only = callTest ./test-only { inherit util; };
 
     unit = unitTests;
   };

test/test-only/Main.hs

@@ -0,0 +1,2 @@
+module Main where
+main = return ()

test/test-only/cabal.project

@@ -0,0 +1 @@
+packages: .

test/test-only/default.nix

@@ -0,0 +1,17 @@
+{ stdenv, lib, util, project', haskellLib, recurseIntoAttrs, testSrc, compiler-nix-name, evalPackages }:
+
+with lib;
+
+let
+  project = project' {
+    inherit compiler-nix-name evalPackages;
+    src = testSrc "test-only";
+  };
+
+in recurseIntoAttrs {
+  ifdInputs = {
+    inherit (project) plan-nix;
+  };
+
+  run = project.hsPkgs.test-only.checks.my-test;
+}

test/test-only/test-only.cabal

@@ -0,0 +1,8 @@
+cabal-version: 1.24
+name: test-only
+version: 1
+build-type: Simple
+test-suite my-test
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  build-depends: base