zcash_client_backend: Fix note selection & add more multi-pool tests. #5013

Job	Run time
Latest build on ubuntu-latest	1m 29s
Test on Linux	9m 5s
Latest build on windows-latest	3m 43s
Check ZFuture build on Linux	26s
Test Orchard on Linux	6m 53s
Latest build on macOS-latest	2m 37s
Check ZFuture build on macOS	1m 0s
Build target wasm32-wasi	1m 19s
Test NU6 on Linux	9m 9s
Check ZFuture build on Windows	1m 22s
Test on macOS	17m 27s
Bitrot check	1m 42s
Test Orchard on macOS	12m 56s
Clippy (MSRV)	1m 56s
Test on Windows	14m 2s
Clippy (beta)	17s
Test Orchard on Windows	10m 57s
Code coverage	16m 57s
Test NU6 on Windows	14m 8s
Intra-doc links	2m 28s
Rustfmt	14s
protobuf consistency	1m 37s
UUID validity	5s
	2h 11m 49s

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