Add test case for generic Partial type

Author: timsuchanek

tests/cases/fourslash/completionsWithOptionalPropertiesGenericPartial.ts

@@ -0,0 +1,13 @@
+/// <reference path="fourslash.ts" />
+// @strict: true
+
+//// interface Foo {
+////     a_a: boolean;
+////     a_b: boolean;
+////     a_c: boolean;
+////     b_a: boolean;
+//// }
+//// function partialFoo<T extends Partial<Foo>>(t: T) {return t}
+//// partialFoo({ /*1*/ });
+
+verify.completions({ marker: '1', includes: ['a_a', 'a_b', 'a_c', 'b_a'] })