Add exhaustive test for String.prototype.substr with various number inputs

Also see: https://github.com/tc39/ecma262/pull/2844

test/annexB/built-ins/String/prototype/substr/start-and-length-as-numbers.js

@@ -0,0 +1,92 @@
+// Copyright (C) 2022 André Bargull. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-string.prototype.substr
+description: >
+  Test String.prototype.substr with number inputs for start and length.
+---*/
+
+function ToIntegerOrInfinity(arg) {
+  assert.sameValue(typeof arg, "number");
+
+  return Number.isNaN(arg) ? 0 : Math.trunc(arg);
+}
+
+// Basic reference implementation. Expects all inputs have the correct type.
+function StringSubstr(string, start, length) {
+  // Steps 1-2.
+  assert.sameValue(typeof string, "string");
+
+  // Step 3.
+  let size = string.length;
+
+  // Step 4.
+  let intStart = ToIntegerOrInfinity(start);
+
+  // Steps 5-7.
+  if (intStart === -Infinity) {
+    intStart = 0;
+  } else if (intStart < 0) {
+    intStart = Math.max(size + intStart, 0);
+  } else {
+    intStart = Math.min(intStart, size)
+  }
+
+  // |0 <= intStart <= size| now holds.
+  assert(0 <= intStart && intStart <= size);
+
+  // Step 8.
+  let intLength = length === undefined ? size : ToIntegerOrInfinity(length);
+
+  // Step 9.
+  intLength = Math.min(Math.max(intLength, 0), size);
+
+  // |0 <= intLength <= size| now holds.
+  assert(0 <= intLength && intLength <= size);
+
+  // Step 10.
+  let intEnd = Math.min(intStart + intLength, size);
+
+  // |intStart <= intEnd <= size| now holds.
+  assert(intStart <= intEnd && intEnd <= size);
+
+  // Step 11.
+  //
+  // Call `substring` and check the result is correct.
+  let result = string.substring(intStart, intEnd);
+
+  assert.sameValue(result.length, intEnd - intStart);
+
+  for (let i = 0; i < result.length; ++i) {
+    assert.sameValue(result[i], string[intStart + i]);
+  }
+
+  return result;
+}
+
+const positiveIntegers = [
+  0, 1, 2, 3, 4, 5, 10, 100,
+];
+
+const integers = [
+  ...positiveIntegers,
+  ...positiveIntegers.map(v => -v),
+];
+
+const numbers = [
+  ...integers,
+  ...integers.map(v => v + 0.5),
+  -Infinity, Infinity, NaN,
+];
+
+for (let string of ["", "a", "ab", "abc"]) {
+  for (let start of numbers) {
+    for (let length of [...numbers, undefined]) {
+      let actual = string.substr(start, length);
+      let expected = StringSubstr(string, start, length);
+
+      assert.sameValue(actual, expected, `"${string}".substr(${start}, ${length})`);
+    }
+  }
+}