Put back explicit setting of replication values within TAP tests.
Commit
151c0c5f7 neglected the possibility that a TEMP_CONFIG file
would explicitly set max_wal_senders=0; as indeed buildfarm member
thorntail does, so that it can test wal_level=minimal in other test
suites. Hence, rather than assuming that max_wal_senders=10 will
prevail if we say nothing, set it explicitly.
Set max_replication_slots=10 explicitly too, just to be safe.
Back-patch to v10, like the previous patch.
Discussion: https://postgr.es/m/723911.
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