Update rappor xml tests and add to test target.

Bug: 772404
Change-Id: I7cb5b15f10aec3db6f28be16d7035bcc7b4d8525
Reviewed-on: https://chromium-review.googlesource.com/1173360Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582783}

tools/metrics/BUILD.gn

@@ -104,6 +104,9 @@ group("metrics_python_tests") {
     "//tools/metrics/histograms/generate_expired_histograms_array_unittest.py",
     "//tools/metrics/histograms/merge_xml.py",
 
+    "//tools/metrics/rappor/model.py",
+    "//tools/metrics/rappor/pretty_print_test.py",
+
     "//tools/metrics/ukm/model.py",
     "//tools/metrics/ukm/pretty_print_test.py",