RFR JDK-8062759: Update test/javax/naming/spi/providers/InitialContextTest.java to use classpath

[Pavel Rappo]

Hi everyone,

Could you please review my change for JDK-8062759?

http://cr.openjdk.java.net/~prappo/8062759/webrev.00/

----------------------------------------------------------------
P.S. The actual change is located at lines [215, 219]. Everything else is just a minor refactoring.

P.P.S. I decided that existing check (InitialContextTest.verifyOutput) of the output for traces of "ServiceLoader" is good enough to verify that in the 'initialContextFactoryAsService' scenario JNDI provider is loaded by ServiceLoader rather than by VersionHelper.loadClass 

Thanks
-Pavel