some love for the testsuite; fixed regressions

tests/overload/tprefer_tygenericinst.nim

@@ -0,0 +1,17 @@
+discard """
+  output: "Version 2 was called."
+  disabled: true
+"""
+
+# bug #2220
+
+type A[T] = object
+type B = A[int]
+
+proc p[X](x: X) =
+  echo "Version 1 was called."
+
+proc p(x: B) =
+  echo "Version 2 was called."
+
+p(B()) # This call reported as ambiguous.