cros_test: Clobber stateful when flashing the DUT.

Latest tests are failing due to lack of disk space in the stateful
partition under cros_sign_backups/. This can hopefully be resolved
by clobbering stateful at every flash.

BUG=chromium:1127992
TEST=unittest
TEST=ran this on chrome's simple chrome bots: crrev.com/c/1863361/57

Change-Id: I832efe1dc71b2f031781e3c5447a989c9686ffcf
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2410706
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Tested-by: Ben Pastene <bpastene@chromium.org>
diff --git a/lib/cros_test.py b/lib/cros_test.py
index 941ba7e..b6dda3d 100644
--- a/lib/cros_test.py
+++ b/lib/cros_test.py
@@ -182,6 +182,7 @@
         xbuddy_path,
         '--board', self._device.board,
         '--disable-rootfs-verification',
+        '--clobber-stateful',
     ]
     cros_build_lib.run(flash_cmd, dryrun=self.dryrun)