[tests] Fix intermittent rpc_net.py failure.
[tests] Fix intermittent rpc_net.py failure.

rpc_net.py would intermittently fail on Travis, probably
due to assuming that two consecutive RPC calls were atomic.
Fix this by only testing that amounts are bounded above and
below rather than equal.

Github-Pull: #12804
Rebased-From: 5a67c0524e5dc98d0e387f189545bc99863916d4

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