Add tests for getContext and appendContext

tests/eval-compare/builtins.appendContext.nix

@@ -0,0 +1,27 @@
+let
+  drv = derivation {
+    name = "fail";
+    builder = "/bin/false";
+    system = "x86_64-linux";
+    outputs = [ "out" "foo" ];
+  };
+
+  path = "${./eval-okay-context-introspection.nix}";
+
+  desired-context = {
+    "${builtins.unsafeDiscardStringContext path}" = {
+      path = true;
+    };
+    "${builtins.unsafeDiscardStringContext drv.drvPath}" = {
+      outputs = [ "foo" "out" ];
+      allOutputs = true;
+    };
+  };
+
+  legit-context = builtins.getContext "${path}${drv.outPath}${drv.foo.outPath}${drv.drvPath}";
+
+  constructed-context = builtins.getContext (builtins.appendContext "" desired-context);
+in [ (builtins.appendContext "foo" {})
+     (legit-context == constructed-context)
+     constructed-context
+   ]

tests/eval-compare/builtins.getContext.nix

@@ -0,0 +1,5 @@
+with builtins;
+
+[ (getContext "foo")
+  (getContext (toFile "foo" "foo contents"))
+]