fix nim js cmp fails at CT (#16473)

tests/misc/tstrtabs.nim

@@ -0,0 +1,20 @@
+discard """
+  targets: "c cpp js"
+"""
+
+import std/strtabs
+
+proc fun()=
+  let ret = newStringTable(modeCaseSensitive)
+  ret["foo"] = "bar"
+
+  doAssert $ret == "{foo: bar}"
+
+  let b = ret["foo"]
+  doAssert b == "bar"
+
+proc main()=
+  static: fun()
+  fun()
+
+main()