Is it possible to run JUnit tests from multiple packages in Eclipse?

Yes, it is possible. The easiest way for me at least is to add a test suite class. It can look like this:

package tests;

import org.junit.runner.RunWith;
import org.junit.runners.Suite;
import org.junit.runners.Suite.SuiteClasses;

import tests.message.ATest;
import tests.validator.BTest;
import tests.validator.CTest;
import tests.validator.DTest;

@RunWith(Suite.class)
@SuiteClasses({ ATest.class, 
        BTest.class, 
        CTest.class, 
        DTest.class })
public class AllTests {

}

This will allow you to test any class that you import no matter what package it is in. To run this in eclipse you just right click the AllTests class and run it as JUnit test. It will then run all the tests you define in @SuiteClasses.

This will work with linked sources as well, I use it all the time.

An other way:

Click on the black triangle denoted by red rectangle in the picture below (in your Eclipse, not here :).)

Then open run configurations, create a new configuration and then set "Run all tests..." as exemplified in the image below.

Maybe not exactly what the original question was, but you can easily run all tests of a whole Project, by simply right-clicking the project -> Run As JUnitTest. Don't worry where the annotated classes reside, this will be scanned.

This does not work if applied to the test-src-folder or a package with subpackes. Quite a shame actually -.-