This test checks for the substring "foo" in the "secret ls" output. This
is a valid base36 substring and can sometimes show up by chance:

docker_cli_secret_create_test.go:86:
c.Assert(out, checker.Not(checker.Contains), name)
... obtained string = "" +
... "ID NAME CREATED UPDATED\n" +
... "ob8y4t4feuz8pn5h6vla9oxoz stefoo7e268ozqfupi9s4se9q Less than a second ago Less than a second ago\n"
... substring string = "foo"

Change the secret name to test_secret, matching other tests. Underscores
can't appear in base36 so this name is safe to grep for.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>