pgsql: Disable one set of tests from c8841199509.

Disable one set of tests from c8841199509.

One of the upsert related tests is unstable (sometimes even hanging
until isolationtester's step timeout is reached). Based on preliminary
analysis that might be a problem outside of just that test, but not
really related to EPQ and triggers.  Disable for now, to get the
buildfarm greener again.

Discussion: https://postgr.es/m/20191004222437.45qmglpto43pd3jb@alap3.anarazel.de
Backpatch: 9.6-, just like c8841199509.

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/bbd26778643f8e353556bce09831f2c3107fb20e

Modified Files
--------------
.../isolation/expected/eval-plan-qual-trigger.out  | 217 ---------------------
.../isolation/specs/eval-plan-qual-trigger.spec    |  21 +-
2 files changed, 11 insertions(+), 227 deletions(-)