Fix flaky subcommand timeout in cros_build_lib_unittest.py.
Fix flaky subcommand timeout in cros_build_lib_unittest.py.

BUG=none
TEST=Run the unit test.

Change-Id: I8c303fbff5362a097e23a933651635fb59373eef
Reviewed-on: https://gerrit.chromium.org/gerrit/46310
Reviewed-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Commit-Queue: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite