Categorize Eclipse sources as main vs. test

This leverages new Eclipse 4.8 (Photon) features that distinguish main
code from test code:
<https://www.eclipse.org/photon/noteworthy/index.php#test-sources>.
An eventual fix to <https://github.com/gradle/gradle/issues/4802>
might mean that Buildship starts doing this automatically, but for now
we can do it ourselves.

If using an older Eclipse release, this change does nothing useful,
but neither does it cause any harm.

build.gradle

@@ -164,3 +164,21 @@ allprojects {
 		options.addStringOption('Xdoclint:none', '-quiet')
 	}
 }
+
+
+////////////////////////////////////////////////////////////////////////
+//
+//  Eclipse IDE integration
+//
+
+// workaround for <https://github.com/gradle/gradle/issues/4802>
+allprojects {
+	apply plugin: 'eclipse'
+
+	eclipse.classpath.file.whenMerged {
+		entries.each {
+			if (it in org.gradle.plugins.ide.eclipse.model.AbstractClasspathEntry && it.entryAttributes['gradle_used_by_scope'] == 'test')
+				it.entryAttributes['test'] = true
+		}
+	}
+}