test: confirm that assert truncates long values

`assert` has code to truncate values that take more than 128 characters
to display. Add a test for this functionality, as the code is not
currently exercised in the existing tests.

PR-URL: https://github.com/nodejs/node/pull/8134
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-assert.js

@@ -528,4 +528,9 @@ testBlockTypeError(assert.doesNotThrow, undefined);
 assert.throws(() => { throw 'error'; }, (err) => err === 'error');
 assert.throws(() => { throw new Error(); }, (err) => err instanceof Error);
 
+// Long values should be truncated for display.
+assert.throws(() => {
+  assert.strictEqual('A'.repeat(1000), '');
+}, new RegExp(`^AssertionError: '${'A'.repeat(127)} === ''$`));
+
 console.log('All OK');