commands: RunCrosSigningTests inside chroot.

CrosSigning tests depend on packages that don't always exist outside
the chroot.

BUG=None
TEST=cbuildbot --remote -g N  signer-pre-cq

Change-Id: Idb2e3d484c5dfc16883cf452795ced7bc9222490
Reviewed-on: https://chromium-review.googlesource.com/411363
Commit-Ready: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/cbuildbot/commands.py b/cbuildbot/commands.py
index a59cce2..9b2ac75 100644
--- a/cbuildbot/commands.py
+++ b/cbuildbot/commands.py
@@ -438,9 +438,9 @@
   Args:
     buildroot: The buildroot of the current build.
   """
-  test_runner = os.path.join(
-      buildroot, 'cros-signing', 'signer', 'run_tests.py')
-  cros_build_lib.RunCommand([test_runner], cwd=buildroot)
+  test_runner = path_util.ToChrootPath(os.path.join(
+      buildroot, 'cros-signing', 'signer', 'run_tests.py'))
+  cros_build_lib.RunCommand([test_runner], enter_chroot=True)
 
 
 def UpdateBinhostJson(buildroot):