RFR: 8215890: Simplify jdk/jfr/api/recording/event/TestPeriod.java

[Erik Gahlin]

Hi,

Could I have a review of this bug fix.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8215890

Webrev:
http://cr.openjdk.java.net/~egahlin/8215890/

Testing:
100 times on a saturated machine.

The test relies on measuring the time a periodic event takes, but there 
is no way for JFR to guarantee that events arrive on time. I have 
instead modified the test, so it checks that at least three events 
arrive. The period has been set picked so a time conversion bug, where 
millis is mixed up with micros/nanos, will make the test timeout.

Thanks
Erik