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