Silence rm errors when no cache files exist

BUG=chromium:703769
TEST=None

Change-Id: I007038454bf5e5c184345393436b81c8f0718ccc
diff --git a/bin/run_tests b/bin/run_tests
index 595036b..092fc63 100755
--- a/bin/run_tests
+++ b/bin/run_tests
@@ -6,7 +6,7 @@
 cd -- "$(readlink -e -- "$(dirname -- "$0")")"
 . ./find_virtualenv.sh
 cd ..
-find . -name "*.pyc" -print0 | xargs -0 rm
+find . -name "*.pyc" -print0 | xargs -0 rm -f
 while getopts v name; do
     case $name in
         v) verbose=1;;