Explicitly set the txn priority in TestPropagateTxnOnPushError
Explicitly set the txn priority in TestPropagateTxnOnPushError

I couldn't reproduce a failure locally by "make stress PKG=./kv", so
this will probably not fix a stress test we saw (#4906), but I wanted
to give a shot.

Also fix comments, clean up the test, and add some logging. The test
was updated with 2a858b06, but most of the comments were not updated.

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