SI-8642 Enable OSGi tests under Java 8

We use the PAX Exam framework to integration test that the
OSGi metadata we add to our JARs allows them to be loaded
into the Felix and Equinox containers. However, we had to
disable this test under Java 8 due to an incompatibility between
that framework and the modern Java version.

I have found a combination that works in Java 6, 7, and 8,
so the test is now run under all Java versions.

I have left a `skip` property to disable them, following the
established convention.

Tip of the hat to:

  - @soc / @rkrzewski for the work in scala#4066 that paved the way
    for this small change
  - Harald Wellman, for sharing [1] the Java 8 compatible combination
    of PAX and Felix .

Testing:

```
for V in 1.6 1.7 1.8; do java_use $V; ant -q test.osgi; done
java version "1.6.0_65"
Java(TM) SE Runtime Environment (build 1.6.0_65-b14-466.1-11M4716)
Java HotSpot(TM) 64-Bit Server VM (build 20.65-b04-466.1, mixed mode)
     ...
     [echo] Running OSGi JUnit tests. Output in /Users/jason/code/scala3/build/osgi
     [echo] Test pass 1 of 2 using Apache Felix 4.4.0
     [echo] Test pass 2 of 2 using Eclipse Equinox 3.7.1

BUILD SUCCESSFUL
Total time: 31 seconds
java version "1.7.0_71"
Java(TM) SE Runtime Environment (build 1.7.0_71-b14)
Java HotSpot(TM) 64-Bit Server VM (build 24.71-b01, mixed mode)
     ...
     [echo] Running OSGi JUnit tests. Output in /Users/jason/code/scala3/build/osgi
     [echo] Test pass 1 of 2 using Apache Felix 4.4.0
     [echo] Test pass 2 of 2 using Eclipse Equinox 3.7.1

BUILD SUCCESSFUL
Total time: 22 seconds
java version "1.8.0_25"
Java(TM) SE Runtime Environment (build 1.8.0_25-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.25-b02, mixed mode)
     ...
     [echo] Running OSGi JUnit tests. Output in /Users/jason/code/scala3/build/osgi
     [echo] Test pass 1 of 2 using Apache Felix 4.4.0
     [echo] Test pass 2 of 2 using Eclipse Equinox 3.7.1

BUILD SUCCESSFUL
Total time: 16 seconds
```

[1] https://groups.google.com/d/msg/ops4j/TN0sZFf6wLs/vUP0GML6-TQJ

build.xml

@@ -280,7 +280,7 @@ TODO:
 
       <!-- Pax runner -->
       <property name="pax.exam.version" value="3.5.0"/><!-- Last version which supports Java 6 -->
-      <property name="osgi.felix.version" value="4.0.3"/>
+      <property name="osgi.felix.version" value="4.4.0"/>
       <property name="osgi.equinox.version" value="3.7.1"/>
       <artifact:dependencies pathId="pax.exam.classpath" filesetId="pax.exam.fileset">
         <dependency groupId="org.ops4j.pax.exam" artifactId="pax-exam-container-native" version="${pax.exam.version}">