[Toybox] [PATCH] chmod.test: make the tests a bit more robust.

[enh]

If a chmod test fails it can leave files/directories in a state
that causes successive runs to suffer bogus failures, and that `make
clean` can't clean up without manual intervention.

Also, macOS seems to disallow +s in /tmp (but not otherwise), so
allow those tests to be skipped. This is why I was seeing macOS
test failures locally (because I pretty much always work on toybox
in /tmp) that we weren't seeing on the github CI runs (which
presumably don't run in /tmp).

With this, all the tests are passing locally for me on macOS again.
---
 tests/chmod.test | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)
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