test: validate error message from buffer.equals

Adding regular expression as an argument to an assert.throws().

PR-URL: #11215
Backport-PR-URL: #13785
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>

Author: Sebastian Roeder

test/parallel/test-buffer-equals.js

@@ -13,4 +13,5 @@ assert.ok(!c.equals(d));
 assert.ok(!d.equals(e));
 assert.ok(d.equals(d));
 
-assert.throws(() => Buffer.alloc(1).equals('abc'));
+assert.throws(() => Buffer.alloc(1).equals('abc'),
+              /^TypeError: Argument must be a Buffer$/);