Add a regression test for completion list in object literal involving inferred obj with optional members (#48910)

tests/cases/fourslash/completionListInObjectLiteral8.ts

@@ -0,0 +1,18 @@
+/// <reference path="fourslash.ts" />
+
+////declare function test<
+////  Variants extends Partial<Record<'hover' | 'pressed', string>>,
+////>(v: Variants): void
+////
+////test({
+////  hover: "",
+////  /**/
+////});
+
+verify.completions({
+  marker: '',
+  exact: [{
+      name: 'pressed',
+      sortText: completion.SortText.OptionalMember
+  }]
+});