test/migration: ensure that leader client is used for storage read check

[calvn]

Fixes the following failure when testing seal migration via raft since it's not guaranteed that core0 will be the leader due to the nature of the election.

    --- FAIL: TestSealMigration_TransitToShamir_Post14/raft (86.29s)
        teststorage_reusable.go:167: raft dir: /tmp/vault-raft-793422364
        teststorage_reusable.go:167: raft dir: /tmp/vault-raft-204126923
        teststorage_reusable.go:167: raft dir: /tmp/vault-raft-352124846
        teststorage_reusable.go:167: raft dir: /tmp/vault-raft-990210357
        teststorage_reusable.go:167: raft dir: /tmp/vault-raft-057345808
        seal_migration_test.go:645: Get https://127.0.0.1:23404/v1/secret/foo: redirects not allowed in these tests