Add test_namespace option to test.py

This allows you to run tests in specific namespace relative to the r8 root namespace
tools/test.py --test_namespace desugar/desugaredlibrary

Change-Id: I0d857ae002d419c6187c15e6220a92a478bf6048
diff --git a/build.gradle b/build.gradle
index 99676ef..54f0898 100644
--- a/build.gradle
+++ b/build.gradle
@@ -1975,6 +1975,10 @@
         include "com/android/tools/r8/internal/**"
     }
 
+    if (project.hasProperty('test_namespace')) {
+        include "com/android/tools/r8/" + project.getProperty('test_namespace') + "/**"
+    }
+
     if (project.hasProperty('tool')) {
         if (project.property('tool') == 'r8') {
             exclude "com/android/tools/r8/jctf/**"