RFR: 8168343: 3 javac tests fail when run on an exploded image

[Jonathan Gibbons]

Please review some small changes to 3 javac tests, to allow them to run 
on an exploded image.

The "official" way to run tests is using a full image (as resulting from 
"make images").
But it is also highly desirable for developers to be able to skip the 
final stage of building
images, and to be able to run tests on an "exploded image" (as resulting 
from "make"
or "make default").

3 langtools tests fail in this case. Two of the failures are because of 
assumptions about
the presence of an image file, and can easily be fixed to accommodate an 
exploded image.
The other test fails because of an issue in the security manager's 
policy files. JDK-8155858.
This test is "fixed" for now by allowing it to be skipped when run on an 
exploded image.

JBS: https://bugs.openjdk.java.net/browse/JDK-8168343
Webrev: http://cr.openjdk.java.net/~jjg/8168343/webrev.00/

-- Jon