Add a better test for jsdoc index signatures.

The test case shows that the errorenous error no longer appears.

tests/cases/conformance/jsdoc/jsdocIndexSignature.ts

@@ -8,3 +8,8 @@ var o1;
 var o2;
 /** @type {Object.<boolean, string>} */
 var o3;
+/** @param {Object.<string, boolean>} o */
+function f(o) {
+    o.foo = 1; // error
+    o.bar = false; // ok
+}