[Tests] Use dedicated SWT native lib cache folder

SWT tests often also involve testing a new snapshot of the native
libraries. As SWT usually puts the libraries in a cache under
user.home, it makes SWT tests more likely to used the cached version
then the one that inside platform-specific fragments.
SWT Tests are now configured to use a different user.home to not interact
with the default cache directory.

Signed-off-by: Mickael Istria <mistria@redhat.com>
diff --git a/tests/org.eclipse.swt.tests/pom.xml b/tests/org.eclipse.swt.tests/pom.xml
index e56e6bd..9b1811a 100644
--- a/tests/org.eclipse.swt.tests/pom.xml
+++ b/tests/org.eclipse.swt.tests/pom.xml
@@ -66,6 +66,9 @@
             <configuration>
               <testClassesDirectory>${project.build.outputDirectory}</testClassesDirectory>
               <argLine>@{tycho.testArgLine} -Dorg.eclipse.swt.internal.gtk.disablePrinting ${os-jvm-flags}</argLine>
+              <systemPropertyVariables>
+                <user.home>${project.build.directory}</user.home> <!-- used as cache directory for SWT native libraries -->
+              </systemPropertyVariables>
             </configuration>
           </execution>
         </executions>