test_renepay.py: use test-specific temp dir, not /tmp

[whitslack]

“Save Our Temp Dir” redux!

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[rustyrussell]

This looks like debugging stuff, can't we just delete those lines?

[Lagrang3]

Those lines are not needed for the tests per se, they were added there for debugging a while ago when I was crafting the test.
They can be removed.