test: add test validating assignability with readonly tuples

Author: som-sm

test-d/fixed-length-array.ts

@@ -4,6 +4,7 @@ import type {FixedLengthArray} from '../index.d.ts';
 type FixedToThreeStrings = FixedLengthArray<string, 3>;
 
 expectAssignable<FixedToThreeStrings>(['a', 'b', 'c']);
+expectAssignable<readonly [string, string, string]>({} as FixedToThreeStrings);
 
 expectType<string>({} as FixedToThreeStrings[0]);
 expectType<string>({} as FixedToThreeStrings[1]);