test: improve test-assert.js

Verify error message thrown from assert.ifError

PR-URL: nodejs#11193
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

test/parallel/test-assert.js

@@ -387,7 +387,8 @@ assert.throws(makeBlock(thrower, TypeError));
                      'a.doesNotThrow is not catching type matching errors');
 }
 
-assert.throws(function() { assert.ifError(new Error('test error')); });
+assert.throws(function() { assert.ifError(new Error('test error')); },
+              /^Error: test error$/);
 assert.doesNotThrow(function() { assert.ifError(null); });
 assert.doesNotThrow(function() { assert.ifError(); });