Delete crash dumps and logs that persist from tests on cros VMs.

The following two dirs can accumulate cruft over a VM's life time:
- /var/log/chrome/
- /var/spool/crash/

This clears them before each test. Should become unnecessary when
crbug.com/878526 is closed out.

Bug: 880878
Change-Id: I5414cb65016294938e9e68db495946bbb05cad48
Reviewed-on: https://chromium-review.googlesource.com/1208254
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589007}

build/chromeos/run_vm_test.py

@@ -168,6 +168,16 @@ def build_test_command(self):
     # Build the shell script that will be used on the VM to invoke the test.
     vm_test_script_contents = ['#!/bin/sh']
 
+    # Clear out directories that persist logs and crash dumps. These can
+    # accumulate over a VM's lifetime and consume disk space.
+    # TODO(crbug.com/878526): Remove this once cros_run_vm_test handles it.
+    vm_test_script_contents += [
+        # We run tests as chronos, but need to be root to rm the files. So pass
+        # in the public plaintext root password to sudo via stdin.
+        'echo "test0000" | sudo -S find /var/spool/crash/ -type f -delete',
+        'echo "test0000" | sudo -S find /var/log/chrome/ -type f -delete',
+    ]
+
     # /home is mounted with "noexec" in the VM, but some of our tools
     # and tests use the home dir as a workspace (eg: vpython downloads
     # python binaries to ~/.vpython-root). /tmp doesn't have this