tests: add a dedicated test

Author: zerbina

tests/lang_objects/objects/tgeneric_base_runtime_of.nim

@@ -0,0 +1,26 @@
+discard """
+  description: '''
+    Tests to make sure the run-time 'of' works correctly when the base type
+    is generic.
+  '''
+  targets: "c js vm"
+"""
+
+type
+  Base[T] = object of RootObj
+    x: T
+  DerivedA = object of Base[int]
+  DerivedB = object of Base[float]
+
+var a: ref RootObj = new(DerivedA)
+var b: ref RootObj = new(DerivedB)
+
+doAssert a of DerivedA
+doAssert a of Base[int]
+doAssert not(a of DerivedB)
+doAssert not(a of Base[float])
+
+doAssert b of DerivedB
+doAssert b of Base[float]
+doAssert not(b of DerivedA)
+doAssert not(b of Base[int])