Add r8mappings for tests to third party dependencies

Change-Id: Iea18ccc31b24ed4851a8fe30b18ecb3c1efcf68f
diff --git a/.gitignore b/.gitignore
index 33bdcfe..b42d835 100644
--- a/.gitignore
+++ b/.gitignore
@@ -135,6 +135,8 @@
 third_party/openjdk/jdk-11-test.tar.gz
 third_party/r8
 third_party/r8.tar.gz
+third_party/r8mappings
+third_party/r8mappings.tar.gz
 third_party/sample_libraries
 third_party/sample_libraries.tar.gz
 src/test/jack/ub-jack