As of 16-August-2018 this appears to be an issue. Note the following apex test method.

@isTest static void myTestMethod() {

    System.Test.StartTest();
        String jobId = System.schedule(xxx, xxx, xxx);
    System.Test.StopTest();

    //Assetion failed. Expected "Finished" actual "Waiting"
    System.assertEquals('Finished', [select State from CronTrigger where Id = :jobId ][0].State);
}

As of 16-August-2018 this appears to be an issue. Note the following apex test method.

@isTest static void myTestMethod() {

    System.Test.StartTest();
        String jobId = System.schedule(xxx, xxx, xxx);
    System.Test.StopTest();

    //Assertion failed. Expected "Finished" actual "Waiting"
    System.assertEquals('Finished', [select State from CronTrigger where Id = :jobId ][0].State);
}