rt_union regress failure in PostgreSQL 10

[robe]

 rt_union .. failed (diff expected obtained: /var/lib/jenkins/workspace/postgis/tmp/2_4_pg10.0w64/test_65_diff)
-----------------------------------------------------------------------------
--- rt_union_expected	2017-02-07 07:21:33.425359382 +0000
+++ /var/lib/jenkins/workspace/postgis/tmp/2_4_pg10.0w64/test_65_out	2017-05-25 19:14:24.161734073 +0000
@@ -53,9 +53,6 @@
 SUM|2|3|2
 SUM|3|3|2
 NOTICE:  No pixels found for band 1
-NOTICE:  No pixels found for band 1
-NOTICE:  No pixels found for band 1
-NOTICE:  No pixels found for band 1
 COUNT|1|1|1
 COUNT|2|1|1
 COUNT|3|1|1
-----------------------------------------------------------------------------

This is the last regress failure left for raster, and caused by the optimization change in PostgreSQL 10 to resolve a set returning function only once.

Even if you do something like

().*

which is very bad practice, but we do it all over the place in raster regress tests.

[robe]

In 15405:

Revise rt_union test so passes equally on PostgreSQL 10 and lower. Lower calls SRF for each column, triggering the same error notice multiple times, PostgreSQL 10 called SRF only once. Revised syntax so old versions call SRF only once.

Closes #3763 for trunk PostGIS 2.4.0