A hibrid of dumper tests (xmlDump) and simple CppUnit tests.
Advantages:
* Easy to add a new test case
** Add a new test file and generate the reference
** Similar to dumper tests
* Easy to find out the root of the problem when test fails
** Assertion is not coming somewhere from an XML file
** Assertion are placed in the code, so if you read and
understand that code, you can find out easier why the test fails.
** Similar to simple CppUnit tests.
* One test checks only one smaller part of the whole document
** e.g. legend, chart data, grid

Change-Id: I7bba5a37efcc62d6358c84acece91963243a914f
Reviewed-on: https://gerrit.libreoffice.org/34154Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>
Tested-by: Tamás Zolnai <tamas.zolnai@collabora.com>