RFR: 8230000: some httpclients testng tests run zero test

[Julia Boes]

Hi,

Please find below a fix of three httpclient tests for which testng 
trivially passed without running any tests.

In LineSubscribersAndSurrogatesTest and LineStreamsAndSurrogatesTest, 
the access modifier of the methods in question was changed to public.

For AbstractThrowingPushPromises, the header was adjusted so that it no 
longer appears as a test. This class sets up an abstract framework and 
the real tests are all implemented in subclasses.

Bug: https://bugs.openjdk.java.net/browse/JDK-8230000

Webrev: http://cr.openjdk.java.net/~dfuchs/jboes/8230000/webrev.01/


Regards,

Julia