RFR 8173577: JShell tests: Some testng tests check nothing

[Robert Field]

Well, indeed they didn't, caused by switches between class-level and 
method-level @Test tags.
That was easy to fix.
But then, when the tests ran, they failed.  A little fix to test 
infrastructure, a little fix to launching code, ...
Please review --

Bug:

     https://bugs.openjdk.java.net/browse/JDK-8173577

Webrev:

     http://cr.openjdk.java.net/~rfield/8173577v0.webrev/

Thanks,
Robert