Fixes https://github.com/eclipse-pde/eclipse.pde/issues/68

For example JUnit Plug-in tests, that usually include all available
bundles (Plug-ins and Fragments) when being launched, often suffer from
messages about unresolved bundles due to unsatisfied environment
constraints (e.g. when having platform specific fragments for multiple
different platforms there are always some unsatisfied on the current
platform).

Only exclude unresolved fragments that have a platform filter that does
not match the current target environment and therefore cannot be
resolved in the launched application. Don't just exclude all unresolved
Plug-ins because in general that isn't what is always wanted.