test: add failing test for EXCLUDE constraint WHERE clause being dropped #997

Job	Run time
parser-tests (deparser)	1m 0s
parser-tests (parser)	57s
parser-tests (plpgsql-deparser)	1m 15s
parser-tests (transform)	5m 45s
parser-tests (plpgsql-parser)	1m 9s
parser-tests (proto-parser)	1m 2s
parser-tests (utils)	1m 4s
parser-tests (pgsql-cli)	1m 0s
parser-tests (traverse)	1m 3s
	14m 15s

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