pgsql: Test EXPLAIN (FORMAT JSON) ... XMLTABLE

Test EXPLAIN (FORMAT JSON) ... XMLTABLE

Also, add an alias to the XMLTABLE expression in an existing test.  This
covers some code in explain.c that wasn't previously covered.

I patched xml_2.out blindly :-(

Discussion: https://postgr.es/m/202401181146.fuoeskfzriq7@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/752533d40fd50de0b09d4b956cc32c38f5df2f05

Modified Files
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src/test/regress/expected/xml.out   | 51 +++++++++++++++++++++++++++++++++----
src/test/regress/expected/xml_1.out | 51 +++++++++++++++++++++++++++++++++----
src/test/regress/expected/xml_2.out | 51 +++++++++++++++++++++++++++++++++----
src/test/regress/sql/xml.sql        |  5 +++-
4 files changed, 142 insertions(+), 16 deletions(-)