safe-rm: fix test for Linux

[n-thumann]

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This PR fixes the test of safe-rm for Linux.
Part of #86422.

Currently, the test fails on Linux (e.g. Debian Bullseye), but runs perfectly fine on macOS:

root@908e57a9f29d:/tmp# brew test safe-rm
==> Testing safe-rm
==> /home/linuxbrew/.linuxbrew/Cellar/safe-rm/1.1.0/bin/safe-rm /tmp/safe-rm-test-20221014-25143-k2k4f5/foo
==> /home/linuxbrew/.linuxbrew/Cellar/safe-rm/1.1.0/bin/safe-rm /tmp/safe-rm-test-20221014-25143-k2k4f5/bar 2>&1
Error: safe-rm: failed
An exception occurred within a child process:
  Minitest::Assertion: Expected: 64
  Actual: 1
[...snip...]

As it turns out, rm (whose return code is simply passed via safe-rm), returns 1 on Linux and 64 on BSD when misusing it. This is expected behavior of safe-rm.

➜  homebrew-core git:(master) uname -rs && rm || echo $?
Darwin 21.6.0
usage: rm [-f | -i] [-dIPRrvWx] file ...
       unlink [--] file
64

root@908e57a9f29d:/tmp# uname -rs && rm || echo $?
Linux 5.10.124-linuxkit
rm: missing operand
Try 'rm --help' for more information.
1

[BrewTestBot]

🤖 A scheduled task has triggered a merge.

[n-thumann]

@dawidd6 would you mind adding the hacktoberfest-accepted label to this? Because the PRs get closed by the bot, they won't be taken into account.