hydra-evaluate-jobset: assert it logs errored constituents properly

t/jobs/constituents-broken.nix

@@ -0,0 +1,19 @@
+with import ./config.nix;
+rec {
+  constituentA = null;
+
+  constituentB = mkDerivation {
+    name = "empty-dir-B";
+    builder = ./empty-dir-builder.sh;
+  };
+
+  mixed_aggregate = mkDerivation {
+    name = "mixed_aggregate";
+    _hydraAggregate = true;
+    constituents = [
+      "constituentA"
+      constituentB
+    ];
+    builder = ./empty-dir-builder.sh;
+  };
+}