pgsql: Fix cross-version upgrade test breakage from commit fe07100e82.

Fix cross-version upgrade test breakage from commit fe07100e82.

In commit fe07100e82, I renamed a couple of functions in
test_dsm_registry to make it clear what they are testing. However,
the buildfarm's cross-version upgrade tests run pg_upgrade with the
test modules installed, so this caused errors like:

ERROR: could not find function "get_val_in_shmem" in file ".../test_dsm_registry.so"

To fix, revert those renames. I could probably get away with only
un-renaming the C symbols, but I figured I'd avoid introducing
function name mismatches. Also, AFAICT the buildfarm's
cross-version upgrade tests do not run the test module tests
post-upgrade, else we'll need to properly version the extension.

Per buildfarm member crake.

Discussion: https://postgr.es/m/aGVuYUNW23tStUYs%40nathan

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/0c2b7174c362d3092eb7eabf0117a8d47c64ce0e

Modified Files
--------------
.../test_dsm_registry/expected/test_dsm_registry.out | 14 +++++++-------
.../modules/test_dsm_registry/sql/test_dsm_registry.sql | 4 ++--
.../modules/test_dsm_registry/test_dsm_registry--1.0.sql | 4 ++--
src/test/modules/test_dsm_registry/test_dsm_registry.c | 8 ++++----
4 files changed, 15 insertions(+), 15 deletions(-)