pgsql: pg_upgrade: Make testing different transfer modes easier

pg_upgrade: Make testing different transfer modes easier

The environment variable PG_TEST_PG_UPGRADE_MODE can be set to
override the default transfer mode for the pg_upgrade tests.
(Automatically running the pg_upgrade tests for all supported modes
would be too slow.)

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/50a97009-8ff9-ca4d-a0f6-6086a6775a5b%40enterprisedb.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b059a2409faf5833b3ba7792e247d6466c9e8090

Modified Files
--------------
src/bin/pg_upgrade/TESTING             | 11 +++++++++++
src/bin/pg_upgrade/t/002_pg_upgrade.pl | 15 ++++++++++++---
2 files changed, 23 insertions(+), 3 deletions(-)