Meson: add tests to validate schema/desktop/appdata files

The flatpak CI template already runs ninja tests for us. Let's use that to ensure the appdata/desktop/schema files are always valid Flatpak:

1/4 validate-appdata                        OK       0.04 s 
2/4 validate-desktop                        OK       0.01 s 
3/4 validate-gschema                        OK       0.01 s 
4/4 test-totem                              OK       0.12 s 

Ok:                    4
Expected Fail:         0
Fail:                  0
Unexpected Pass:       0
Skipped:               0
Timeout:               0

Fedora:

1/8 validate-appdata                        OK       0.04 s 
2/8 validate-desktop                        OK       0.02 s 
3/8 validate-gschema                        OK       0.02 s 
4/8 test-totem                              OK       0.29 s 
5/8 pylint-dbusservice                      OK       1.59 s 
6/8 pylint-opensubtitles                    OK      20.82 s 
7/8 pylint-pythonconsole                    OK      21.42 s 
8/8 pylint-samplepython                     OK       1.03 s 

Ok:                    8
Expected Fail:         0
Fail:                  0
Unexpected Pass:       0
Skipped:               0
Timeout:               0

I would like to get this in before merging !109 (merged)