Basically working. Checking against actual use cases.

pkgs/development/ruby-modules/bundler-env/test.nix

@@ -1,9 +1,17 @@
+/*
+Run with:
+nix-build -E 'with import <nixpkgs> { }; callPackage ./test.nix {}' --show-trace; and cat result
+
+Confusingly, the ideal result ends with something like:
+error: build of ‘/nix/store/3245f3dcl2wxjs4rci7n069zjlz8qg85-test-results.tap.drv’ failed
+*/
 { writeText, lib, ruby, defaultGemConfig, callPackage }:
 let
   test = import ./testing.nix;
   tap = import ./tap-support.nix;
 
   bundlerEnv = callPackage ./default.nix {};
+  basicEnv = callPackage ./basic.nix {};
 
   testConfigs = {
     groups = ["default"];
@@ -22,6 +30,18 @@ let
     if builtins.isAttrs actual then
     (test.passed "is a set") else
     (test.failed "is not a set, was ${builtins.typeOf actual}: ${toString actual}");
+
+    haveKeys = expected: actual:
+    if builtins.all
+      (ex: builtins.any (ac: ex == ac) (builtins.attrNames actual))
+      expected then
+      (test.passed "has expected keys") else
+      (test.failed "keys differ: expected [${lib.concatStringsSep ";" expected}] have [${lib.concatStringsSep ";" (builtins.attrNames actual)}]");
+
+    havePrefix = expected: actual:
+    if lib.hasPrefix expected actual then
+    (test.passed "has prefix '${expected}'") else
+    (test.failed "prefix '${expected}' not found in '${actual}'");
   };
 
   justName = bundlerEnv {
@@ -29,9 +49,12 @@ let
     gemset = ./test/gemset.nix;
   };
 
-  pnamed = bundlerEnv {
+  pnamed = basicEnv {
     pname = "test";
+    gemdir = ./test;
     gemset = ./test/gemset.nix;
+    gemfile = ./test/Gemfile;
+    lockfile = ./test/Gemfile.lock;
   };
 
   results = builtins.concatLists [
@@ -40,10 +63,14 @@ let
       name = should.equal "test";
     })
     (test.run "bundlerEnv { pname }" pnamed
-     {
-      name = should.equal "test-0.1.2";
-      env = should.beASet;
-    })
+    [
+      (should.haveKeys [ "name" "env" "postBuild" ])
+      {
+        name = should.equal "test-0.1.2";
+        env = should.beASet;
+        postBuild = should.havePrefix "nananana";
+      }
+    ])
   ];
 in
   writeText "test-results.tap" (tap.output results)