类：https://github.com/Keanor/generatortest/blob/master/src/SomeClass.php
测试：https://github.com/Keanor/generatortest/blob/master/tests/SomeClassTest.php
输出：
keanor@keanor-pc ~/www/generatortest $ ./vendor/bin/phpunit
PHPUnit 5.3.2 by Sebastian Bergmann and contributors.
F 1 / 1 (100%)class Generator#23 (0) {
}
Time: 31 ms, Memory: 3.75Mb
There was 1 failure:
1) AppTest'SomeClassTest::testMethod2
Expectation failed for method name is equal to <string:method1> when invoked 1 time(s).
Method was expected to be called 1 times, actually called 0 times.
FAILURES!
Tests: 1, Assertions: 1, Failures: 1.
如果将"yield"替换为"return"，则测试成功！
Mock不适用于发电机？
如果你像生成器一样使用它，它就会起作用：
$mock = $this->getMockBuilder(SomeClass::class)
->setMethods(['method1'])
->getMock();
$mock->expects($this->once())
->method('method1')
->willReturn('');
foreach ($mock->method2() as $result) {
var_dump($result);
}
而不是：
$mock = $this->getMockBuilder(SomeClass::class)
->setMethods(['method1'])
->getMock();
$mock->expects($this->once())
->method('method1')
->willReturn('');
$result = call_user_func([$mock, 'method2']);
var_dump($result);