pgsql: Trim regression test expected output for xml

Trim regression test expected output for xml

This commit reduces the number of expected output files for the "xml"
test from three to two (well, mostly one, see below for details).

xml_2.out existed to handle some differences in output due to libxml2
2.9.3, due to some error context missing (085423e3e326). This file is
removed, by tweaking the XML inputs to trigger the same error patterns
for the problematic 2.9.3 and other libxml2 versions. This part is
authored by Tom Lane.

xml_1.out (no libxml2 support) is reduced in size by adding an \if query
that exits the test early. This still checks NO_XML_SUPPORT() through
xmlin(). The rest of the test is skipped if XML input cannot be
handled by the backend. This part has been written by me.

Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Author: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Discussion: https://postgr.es/m/aiu6CXO67q-s70n5@paquier.xyz
Backpatch-through: 14

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/f7fc53d938689668529c33f2830c066900a99082

Modified Files
--------------
src/test/regress/expected/xml.out | 56 +-
src/test/regress/expected/xml_1.out | 1426 +---------------------------
src/test/regress/expected/xml_2.out | 1795 -----------------------------------
src/test/regress/sql/xml.sql | 21 +-
4 files changed, 52 insertions(+), 3246 deletions(-)