tests: dashdemux: add unit tests for checking the parsing of MPD element

tests: dashdemux: add unit tests for checking the parsing of MPD element

Create a dash test target and add unit tests that check the parsing of
attributes of the MPD element and the following child elements:
 - baseURL
 - program information
 - location
 - metrics

These tests check the compliance of parts of the MPD parser in the
dashdemux element against the DASH specification [1].

    [1] http://standards.iso.org/ittf/PubliclyAvailableStandards/c065274_ISO_IEC_23009-1_2014_Electronic_inserts.zip
        http://standards.iso.org/ittf/PubliclyAvailableStandards/c065274_ISO_IEC_23009-1_2014.zip

https://bugzilla.gnome.org/show_bug.cgi?id=750390