nix-lang parser: Add mixed nested attrs tests.

Part of issue #2077

tests/lang/parse-fail-mixed-nested-attrs.nix

@@ -0,0 +1,4 @@
+{ 
+  x = { y = 3; z = 3; }; 
+  x.z = 3; 
+}

tests/lang/parse-okay-mixed-nested-attrs-1.nix

@@ -0,0 +1,4 @@
+{ 
+  x = { y = 3; z = 3; }; 
+  x.q = 3; 
+}

tests/lang/parse-okay-mixed-nested-attrs-2.nix

@@ -0,0 +1,4 @@
+{ 
+  x.q = 3; 
+  x = { y = 3; z = 3; }; 
+}

tests/lang/parse-okay-nested-attrs.nix

@@ -0,0 +1,4 @@
+{ 
+  x = { y = 3; z = 3; }; 
+  x.q = 3; 
+}