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

src/test/modules/test_dsm_registry/expected/test_dsm_registry.out

@@ -1,7 +1,7 @@
 CREATE EXTENSION test_dsm_registry;
-SELECT set_val_in_dsm(1236);
- set_val_in_dsm 
-----------------
+SELECT set_val_in_shmem(1236);
+ set_val_in_shmem 
+------------------
  
 (1 row)
 
@@ -12,10 +12,10 @@ SELECT set_val_in_hash('test', '1414');
 (1 row)
 
 \c
-SELECT get_val_in_dsm();
- get_val_in_dsm 
-----------------
-           1236
+SELECT get_val_in_shmem();
+ get_val_in_shmem 
+------------------
+             1236
 (1 row)
 
 SELECT get_val_in_hash('test');

src/test/modules/test_dsm_registry/sql/test_dsm_registry.sql

@@ -1,6 +1,6 @@
 CREATE EXTENSION test_dsm_registry;
-SELECT set_val_in_dsm(1236);
+SELECT set_val_in_shmem(1236);
 SELECT set_val_in_hash('test', '1414');
 \c
-SELECT get_val_in_dsm();
+SELECT get_val_in_shmem();
 SELECT get_val_in_hash('test');

src/test/modules/test_dsm_registry/test_dsm_registry--1.0.sql

@@ -3,10 +3,10 @@
 -- complain if script is sourced in psql, rather than via CREATE EXTENSION
 \echo Use "CREATE EXTENSION test_dsm_registry" to load this file. \quit
 
-CREATE FUNCTION set_val_in_dsm(val INT) RETURNS VOID
+CREATE FUNCTION set_val_in_shmem(val INT) RETURNS VOID
 	AS 'MODULE_PATHNAME' LANGUAGE C;
 
-CREATE FUNCTION get_val_in_dsm() RETURNS INT
+CREATE FUNCTION get_val_in_shmem() RETURNS INT
 	AS 'MODULE_PATHNAME' LANGUAGE C;
 
 CREATE FUNCTION set_val_in_hash(key TEXT, val TEXT) RETURNS VOID

src/test/modules/test_dsm_registry/test_dsm_registry.c

@@ -70,9 +70,9 @@ tdr_attach_shmem(void)
 		tdr_hash = GetNamedDSHash("test_dsm_registry_hash", &dsh_params, &found);
 }
 
-PG_FUNCTION_INFO_V1(set_val_in_dsm);
+PG_FUNCTION_INFO_V1(set_val_in_shmem);
 Datum
-set_val_in_dsm(PG_FUNCTION_ARGS)
+set_val_in_shmem(PG_FUNCTION_ARGS)
 {
 	tdr_attach_shmem();
 
@@ -83,9 +83,9 @@ set_val_in_dsm(PG_FUNCTION_ARGS)
 	PG_RETURN_VOID();
 }
 
-PG_FUNCTION_INFO_V1(get_val_in_dsm);
+PG_FUNCTION_INFO_V1(get_val_in_shmem);
 Datum
-get_val_in_dsm(PG_FUNCTION_ARGS)
+get_val_in_shmem(PG_FUNCTION_ARGS)
 {
 	int			ret;