* ldtp/fields-edit/fields-edit.py: Added a new test which tests adding
	a new field to an existing table. More operations to test, such as
	changing and removing existing fields, can be added later.

	* ldtp/fields-edit/postgres-central.xml:
	* ldtp/fields-edit/postgres-self.xml:
	* ldtp/fields-edit/sqlite.xml: XML Data files for the new test.

	* ldtp/test.xml: Added it to the test suite, so that it can be run
	with ldtprunner.