pgsql: Fix broken wait-for-previous-process-to-exit loop in regression

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.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/19fcc0058ecc8e5eb756547006bc1b24a93cbb80

Modified Files
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src/test/modules/test_extensions/expected/test_extensions.out | 2 +-
src/test/modules/test_extensions/sql/test_extensions.sql      | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)