Remove incidental md5() function use from test

Replace md5() with sha256() in tests introduced in 14ffaece0f, to
allow test to pass in OpenSSL FIPS mode.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/3518736.1743307492@sss.pgh.pa.us

contrib/amcheck/expected/check_gin.out

@@ -50,9 +50,9 @@ SELECT setseed(1);
 
 CREATE TABLE "gin_check_text_array"("Column1" text[]);
 -- posting trees
-INSERT INTO gin_check_text_array select array_agg(md5(round(random()*300)::text)::text) from generate_series(1, 100000) as i group by i % 10000;
+INSERT INTO gin_check_text_array select array_agg(sha256(round(random()*300)::text::bytea)::text) from generate_series(1, 100000) as i group by i % 10000;
 -- posting leaves
-INSERT INTO gin_check_text_array select array_agg(md5(round(random()*300 + 300)::text)::text) from generate_series(1, 10000) as i group by i % 100;
+INSERT INTO gin_check_text_array select array_agg(sha256(round(random()*300 + 300)::text::bytea)::text) from generate_series(1, 10000) as i group by i % 100;
 CREATE INDEX gin_check_text_array_idx on "gin_check_text_array" USING GIN("Column1");
 SELECT gin_index_check('gin_check_text_array_idx');
  gin_index_check 

contrib/amcheck/sql/check_gin.sql

@@ -30,9 +30,9 @@ DROP TABLE gin_check;
 SELECT setseed(1);
 CREATE TABLE "gin_check_text_array"("Column1" text[]);
 -- posting trees
-INSERT INTO gin_check_text_array select array_agg(md5(round(random()*300)::text)::text) from generate_series(1, 100000) as i group by i % 10000;
+INSERT INTO gin_check_text_array select array_agg(sha256(round(random()*300)::text::bytea)::text) from generate_series(1, 100000) as i group by i % 10000;
 -- posting leaves
-INSERT INTO gin_check_text_array select array_agg(md5(round(random()*300 + 300)::text)::text) from generate_series(1, 10000) as i group by i % 100;
+INSERT INTO gin_check_text_array select array_agg(sha256(round(random()*300 + 300)::text::bytea)::text) from generate_series(1, 10000) as i group by i % 100;
 CREATE INDEX gin_check_text_array_idx on "gin_check_text_array" USING GIN("Column1");
 SELECT gin_index_check('gin_check_text_array_idx');