[Bug] Temp file use in t0018.6

In the 2.46.0 test suite on NonStop I'm getting the following surprise error:

expecting success of 0018.6 'advice should be printed when GIT_ADVICE is set
to true':
        q_to_tab >expect <<-\EOF &&
        On branch trunk

        No commits yet

        Untracked files:
          (use "git add <file>..." to include in what will be committed)
        QREADME

        nothing added to commit but untracked files present (use "git add"
to track)
        EOF

        test_when_finished "rm -fr advice-test" &&
        git init advice-test &&
        (
                cd advice-test &&
                >README &&
                GIT_ADVICE=true git status
        ) >actual &&
        cat actual > /tmp/actual &&
        test_cmp expect actual

Initialized empty Git repository in /home/randall/git-clar/t/trash directory.t0018-advice/advice-test/.git/ ./test-lib.sh: line 1071: /tmp/actual: Permission denied not ok 6 - advice should be printed when GIT_ADVICE is set to true

This is the first I'm seeing a failure I'm seeing of this kind. We should be using randomized temp locations, not fixed. What is going on is that our main CI system runs under one user id while I was running another test under my own user. Our /tmp is configured so that users who create files have exclusive access to those files, regardless of security so /tmp/actual in this case can only be used in one run by one user. What we should be doing is probably generating /tmp/actual.XXXXXXX (randomized) or using actual in the trash directory as normal. In any event, t0018 should be cleaning up and removing /tmp/actual when done. I would suggest doing something like this as a start: