RFR JDK-8250839: Improve test template SSLEngineTemplate with SSLContextTemplate

[Xuelei Fan]

Hi,

May I get the following test code update reviewed?
     http://cr.openjdk.java.net/~xuelei/8250839/webrev.00/

SSLEngineTemplate is a template used for SSLEngine testing, which 
depends on binary key store files, and not easy to extend.  This update 
makes it easier to extend, by removing the files dependency and using 
SSLContextTemplate.

Thanks,
Xuelei