instead of the artificial (to be removed) unit
'org.eclipse.equinox.executable'.
Because the p2.mirror-task that sets up the test environment in the
test.xml is configured with 'followOnlyFilteredRequirements="true"' the
redirection mechanism of the new definition of
'org.eclipse.equinox.executable' does not work.

Additionally print where the executable was found, just for information.

Fixes https://github.com/eclipse-pde/eclipse.pde/issues/1884