zcash_client_backend: Fix note selection & add more multi-pool tests. #5011

Job	Run time
Check ZFuture build on Linux	28s
Test on Linux	9m 4s
Check ZFuture build on macOS	47s
Bitrot check	1m 41s
Test Orchard on Linux	6m 45s
Latest build on ubuntu-latest	1m 30s
Check ZFuture build on Windows	1m 26s
Latest build on windows-latest	4m 53s
Test NU6 on Linux	9m 21s
Test on macOS	18m 33s
Latest build on macOS-latest	1m 59s
Build target wasm32-wasi	1m 26s
Clippy (MSRV)	1m 49s
Test Orchard on macOS	12m 45s
Test on Windows	12m 40s
Code coverage	17m 38s
Test Orchard on Windows	11m 5s
Clippy (beta)	18s
Intra-doc links	2m 22s
Test NU6 on Windows	14m 0s
Rustfmt	13s
protobuf consistency	1m 41s
UUID validity	6s
	2h 12m 30s

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