pgsql: backpatch "Set application_name per-test in isolation and ecpg t

backpatch "Set application_name per-test in isolation and ecpg tests."

We started to backpatch test infrastructure improvements more aggressively to
make it easier to backpatch test. A proposed isolationtester improvement has a
dependency on b1907d688, backpatch b1907d688 to make it easier to subsequently
backpatch the new proposed isolationtester change.

Discussion: https://postgr.es/m/861977.1639421872@sss.pgh.pa.us
Backpatch: 10-12, the commit already is in 13-HEAD

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/fe490f9ad7ab3fc9279c9810304b524aae36c3b6

Modified Files
--------------
src/interfaces/ecpg/test/pg_regress_ecpg.c | 8 ++++++++
src/test/isolation/isolation_main.c | 7 +++++++
src/test/regress/pg_regress.c | 4 ++++
3 files changed, 19 insertions(+)