MultiBundleClassLoader should only load classes via the given bundles,
not directly from the AppClassLoader.

This prevents classloading issues when JUnit bundles are present on the
app classpath.

Change-Id: I5666703b21f4f7d48c67edc9c08b4ca9e732ce44
Reviewed-on: https://git.eclipse.org/r/c/pde/eclipse.pde.ui/+/186815
Tested-by: PDE Bot <pde-bot@eclipse.org>
Reviewed-by: Julian Honnen <julian.honnen@vector.com>