Improve BRIN minmax-multi opclass test coverage

Per the code coverage report, the existing regression tests did not
exercice some a couple important BRIN minmax-multi code paths.

- The tests focused on testing planning with a range of scan key
  strategies, but not the execution. Fixed by adding queries that
  actually test query execution for both equality and inequality.

- All tests created indexes after inserting data, but this only
  exercises the CREATE INDEX strategy that sees all values at once, not
  incremental summary updates. The new tests flip the order and create
  the index before adding data.

- The assert check(s) validating correctness of expanded ranges were
  present only in the "union" code path, which is not covered by
  regression tests at all (as it requires concurrency etc.). Fixed by
  adding the asserts to a couple more places.

Reviewed-by: Heikki Linnakangas
Discussion: https://postgr.es/m/57020b2e-d9c9-9bc7-4892-b36d9bb07563%40enterprisedb.com

src/backend/access/brin/brin_minmax_multi.c

@@ -1660,6 +1660,9 @@ ensure_free_space_in_buffer(BrinDesc *bdesc, Oid colloid,
 	/* build the expanded ranges */
 	eranges = build_expanded_ranges(cmpFn, colloid, range, &neranges);
 
+	/* Is the expanded representation of ranges correct? */
+	AssertCheckExpandedRanges(bdesc, colloid, attno, attr, eranges, neranges);
+
 	/* and we'll also need the 'distance' procedure */
 	distanceFn = minmax_multi_get_procinfo(bdesc, attno, PROCNUM_DISTANCE);
 
@@ -1675,6 +1678,9 @@ ensure_free_space_in_buffer(BrinDesc *bdesc, Oid colloid,
 									  range->maxvalues * MINMAX_BUFFER_LOAD_FACTOR,
 									  cmpFn, colloid);
 
+	/* Is the result of reducing expanded ranges correct? */
+	AssertCheckExpandedRanges(bdesc, colloid, attno, attr, eranges, neranges);
+
 	/* Make sure we've sufficiently reduced the number of ranges. */
 	Assert(count_values(eranges, neranges) <= range->maxvalues * MINMAX_BUFFER_LOAD_FACTOR);
 
@@ -2859,6 +2865,9 @@ brin_minmax_multi_union(PG_FUNCTION_ARGS)
 									  ranges_a->maxvalues,
 									  cmpFn, colloid);
 
+	/* Is the result of reducing expanded ranges correct? */
+	AssertCheckExpandedRanges(bdesc, colloid, attno, attr, eranges, neranges);
+
 	/* update the first range summary */
 	store_expanded_ranges(ranges_a, eranges, neranges);