[cdt-test-dev] testplugin packages

[Andrew Niefer <aniefer@xxxxxxxxxx>]

There has been a conflict between
core.tests/suite/org.eclipse.cdt.testplugin and
ui.tests/src/org.eclipse.cdt.testplugin which caused the core tests to
sometimes incorrectly load the  CTestPlugin class from the ui.tests package
instead of the core.tests package.

As a fix for this, I have refactored the above packages to
org.eclipse.cdt.core.testplugin and org.eclipse.cdt.ui.testplugin.
Similarly for the testplugin.util package.

-Andrew