test: add test case for literal template & fix literalunion test

test-d/join-union.ts

@@ -10,7 +10,9 @@ expectAssignable<''>({} as JoinUnion<null>);
 expectAssignable<''>({} as JoinUnion<undefined>);
 expectAssignable<','>({} as JoinUnion<undefined | null>);
 expectAssignable<'2,'>({} as JoinUnion<undefined | 2>);
-expectAssignable<string>({} as JoinUnion<'foo' | string>); // Intended `foo,${string}`
+
+expectAssignable<`foo,on${string}` | `on${string},foo`>({} as JoinUnion<'foo' | `on${string}`>);
+expectAssignable<`${string & {}}`>({} as JoinUnion<'foo' | (string & {})>); // Intended `foo,${string}`
 // TODO: For now `UnionToTuple` does not handle 'LiteralUnions'. Will be fixed after `ExtractLiterals` type get approved.
 
 expectType<JoinUnion<never>>('');