Merge #12804: [tests] Fix intermittent rpc_net.py failure.

Summary:
5a67c0524e [tests] Fix intermittent rpc_net.py failure. (John Newbery)

Pull request description:

rpc_net.py would intermittently fail on Travis, probably
due to assuming that two consecutive RPC calls were atomic.
Fix this by trying the test up to three times and only failing
if all three attempts fail.

fixes #11778

Tree-SHA512: f2f5047e05114ad2110377e6221ce057680c240952971fb33863834587d2250896c6b697ba27eef739cd0ab23faf47dfae8cafadc9833cbfab5a6f7e36dae5e2

Backport of Core PR12804
https://github.com/bitcoin/bitcoin/pull/12804/files

Test Plan: test_runner.py rpc_net

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4966