cros_build_lib: convert CommandResult to CompletedProcess

Since CommandResult is just CompletedProcess at this point, switch
away from that to better match subprocess APIs.

BUG=b:187789262
TEST=CQ passes

Change-Id: I888ef8c27d8a57b6461deade4ff57846c6e8b04e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3818209
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
diff --git a/api/controller/firmware_unittest.py b/api/controller/firmware_unittest.py
index adf8bf1..e84dbb9 100644
--- a/api/controller/firmware_unittest.py
+++ b/api/controller/firmware_unittest.py
@@ -26,7 +26,7 @@
     self.cros_build_run_patch = self.PatchObject(
         cros_build_lib,
         'run',
-        return_value=cros_build_lib.CommandResult(returncode=0))
+        return_value=cros_build_lib.CompletedProcess(returncode=0))
 
   def _GetInput(self,
                 chroot_path=None,