desktop: make validate-desktop a unit test.

Instead of a target, let's make it a test. Also I realize it already
existed as desktop_file test. But it's simpler to run it directly (no
need of an external script).

Note: I still leave the test-desktop.sh script as it is used for the
autotools test.

desktop/meson.build

@@ -91,8 +91,10 @@ metainfofile = custom_target(metainfofilename,
 )
 
 if desktop_validate.found()
-  run_target('validate-desktop',
-    command: [ desktop_validate, desktopfile ],
+  test('validate-desktop',
+    desktop_validate,
+    args: [ desktopfile ],
+    suite: 'desktop',
   )
 endif
 
@@ -114,12 +116,4 @@ if appstream_util.found()
     ],
     suite: 'desktop',
   )
-
-  test('desktop_file',
-    find_program('test-desktop.sh'),
-    env: [
-      'GIMP_TESTING_ABS_TOP_BUILDDIR=' + meson.build_root(),
-    ],
-    suite: 'desktop',
-  )
 endif