desktop: fix AppStream validation with TODO release date.

As I recall, this is something we in fact used to have in gimp-2-10
branch and which we lost when switching from appstream-util to
appstreamcli validation tool (the later is now recommended, but it's
also less featureful, they don't even have a relax test!). So that was a
regression in the build system.

Note that it's not just not to have to put a date too early in our
AppStream metadata, but it's also that this way, we won't forget to
update it upon release (hence shipping with a wrong release data in
metadata). Indeed, when building a non-release version, date="TODO" will
pass the test, yet moving on to a point release version, the validation
test will now fail on a TODO date.

desktop/meson.build

@@ -45,13 +45,11 @@ if desktop_validate.found()
 endif
 
 if appstreamcli.found()
-   # TODO: should we use validate-strict when the last errors for a strict
-   # validation are fixed?
   test('appdata_file',
-    appstreamcli,
+    find_program('test-appdata.sh', required: true, dirs: [meson.current_source_dir()]),
-    args: [ 'validate', appdatafile ],
     env: [
-      'GIMP_TESTING_ABS_TOP_BUILDDIR=' + meson.project_build_root(),
+      'GIMP_TESTING_BUILDDIR=' + meson.current_build_dir(),
+      'GIMP_RELEASE=' + (release ? '1' : '0'),
     ],
     suite: 'desktop',
   )

desktop/test-appdata.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+
+cd $GIMP_TESTING_BUILDDIR
+if [ $GIMP_RELEASE -eq 1 ]; then
+  appstreamcli validate org.gimp.GIMP.appdata.xml
+  exit $?
+else
+  APPDATA=`mktemp org.gimp.GIMP.appdata.XXX.xml`
+  sed "s/date=\"TODO\"/date=\"`date --iso-8601`\"/" org.gimp.GIMP.appdata.xml > $APPDATA
+  appstreamcli validate $APPDATA
+  success=$?
+  rm $APPDATA
+  exit $success
+fi