This update is to fix `ResourceWarning` messages while running tox
tests because file descriptors are not closed explicitly after each
of tests. Although these tests are completed as `succeeded`, such nouse
warnings make hard to inspect log messages.
Closes-Bug: #1881416
Change-Id: Icff5315969a1303fcbf71ecddfaa7ea0c99a3e5b
Signed-off-by: Yasufumi Ogawa <yasufum.o@gmail.com>