Add unit test for ReplayStage::maybe_start_leader() #2798

Job	Run time
check	0s
test	0s
cargo-test-sbf (token/program)	21m 28s
cargo-test-sbf (instruction-padding/program, token/program-2022, token/program-2022-test)	27m 30s
cargo-test-sbf (associated-token-account/program, associated-token-account/program-test)	14m 42s
cargo-test-sbf (token-upgrade/program)	14m 9s
cargo-test-sbf (feature-proposal/program)	17m 47s
cargo-test-sbf (governance/addin-mock/program, governance/program)	20m 32s
cargo-test-sbf (memo/program)	15m 30s
cargo-test-sbf (name-service/program)	14m 57s
cargo-test-sbf (stake-pool/program)	20m 36s
cargo-test-sbf (single-pool/program)	16m 15s
	3h 3m 26s

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