Fix broken wait-for-previous-process-to-exit loop in regression test.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 19fcc0058ecc8e5eb756547006bc1b24a93cbb80
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2016-12-02T22:23:54Z
Releases: 10.0
Fix broken wait-for-previous-process-to-exit loop in regression test.

Must do pg_stat_clear_snapshot() inside test's loop, or our snapshot of
pg_stat_activity will never change :-(.  Thinko in b3427dade -- evidently
my workstation never really iterated the loop in testing.  Per buildfarm.

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