Add test case

tests/stdlib/tstrformat.nim

@@ -0,0 +1,13 @@
+discard """
+    action: "run"
+"""
+
+import strformat
+
+type Obj = object
+
+proc `$`(o: Obj): string = "foobar"
+
+var o: Obj
+doAssert fmt"{o}" == "foobar"
+doAssert fmt"{o:10}" == "foobar    "