Add default message to x.fail(), update related test

Author: Juan Soto

lib/assert.js

@@ -30,6 +30,7 @@ x.pass = function (msg) {
 };
 
 x.fail = function (msg) {
+	msg = msg || 'Test failed via t.fail()';
 	test(false, create(false, false, 'fail', msg, x.fail));
 };
 

test/api.js

@@ -66,7 +66,7 @@ test('fail-fast mode', function (t) {
 			t.ok(api.options.failFast);
 			t.is(api.passCount, 1);
 			t.is(api.failCount, 1);
-			t.true(/false fail false/.test(api.errors[0].error.message));
+			t.true(/Test failed via t.fail()/.test(api.errors[0].error.message));
 		});
 });