zcash_client_backend: Fix note selection & add more multi-pool tests. #5010

Job	Run time
Check ZFuture build on Linux	23s
Bitrot check	1m 39s
Check ZFuture build on macOS	55s
Build target wasm32-wasi	1m 30s
Latest build on ubuntu-latest	1m 37s
Check ZFuture build on Windows	1m 12s
Latest build on windows-latest	3m 19s
Clippy (MSRV)	2m 8s
Latest build on macOS-latest	2m 15s
Clippy (beta)	11s
Code coverage	17m 41s
Test on Linux	9m 20s
Intra-doc links	2m 27s
Rustfmt	13s
protobuf consistency	1m 36s
Test Orchard on Linux	6m 51s
UUID validity	5s
Test NU6 on Linux	9m 10s
Test on macOS	19m 55s
Test Orchard on macOS	10m 56s
Test on Windows	13m 51s
Test Orchard on Windows	11m 6s
Test NU6 on Windows	13m 39s
	2h 11m 59s

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