test: add assert test for position indicator

This test adds coverage for a ternary in assertion_error.js that checks
if stderr is a TTY.

Author: Trott

test/pseudo-tty/test-assert-position-indicator.js

@@ -0,0 +1,18 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+
+process.env.NODE_DISABLE_COLORS = true;
+process.stderr.columns = 20;
+
+// Confirm that there is no position indicator.
+assert.throws(
+  () => { assert.deepStrictEqual('a'.repeat(30), 'a'.repeat(31)); },
+  (err) => !err.message.includes('^')
+);
+
+// Confirm that there is a position indicator.
+assert.throws(
+  () => { assert.deepStrictEqual('aaa', 'aaaa'); },
+  (err) => err.message.includes('^')
+);