Use pde.junit runtime from target to allow e4.5 tests to run on Eclipse 4.6

Use pde.junit runtime from target to allow e4.5 tests to run on Eclipse 4.6

Fixes trying to run tests with Eclipse 4.6 targetting Eclipse 4.5 using Java 7.
Whitout this change, the runtime from Eclipse 4.6 gets picked up and it
requires Java 8.

Change-Id: I8597ee30fd094bad5b004e21d3fa5b939ca49b7a
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/56563
Reviewed-by: Hudson CI
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Tested-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>