test: cleanup replication/gc

 - Before checking that old WAL files have been removed, wait for the
   garabe collection to remove them to avoid a spurious test failure.
   Currently, the test waits until old checkpoints are removed, but the
   garbage collector can now remove checkpoints, but keep WAL files.
   This is a follow-up for commit 9c5d851d ("replication: remove old
   snapshot files not needed by replicas").
 - Remove a few pointless box.info.gc().checkpoints checks.
 - Use test_run.wait_cond for waiting instead of while-do-sleep loops.