[backport#15415] [test] functional: allow custom cwd, use tmpdir as default

Summary:
[test] functional: set cwd of nodes to tmpdir (Sjors Provoost)

Pull request description:

Any process launched by bitcoind will have `self.datadir` as its `cwd`.

https://github.com/bitcoin/bitcoin/pull/15415/commits/e3e1a5631e2456bb40844862ef35c07c1d1c47b0

Depends on D7080

Backport of Core PR15415

Test Plan:

ninja
test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7081