tests: Improve reliability.
* xgettext-3: Verify the exit code of the xgettext invocation.
* xgettext-4: Likewise.
* xgettext-appdata-1: Likewise.
* xgettext-c-19: Likewise.
* xgettext-c-20: Likewise.
* xgettext-c-21: Likewise.
* xgettext-csharp-2: Likewise.
* xgettext-csharp-3: Likewise.
* xgettext-csharp-8: Likewise.
* xgettext-desktop-1: Likewise.
* xgettext-glade-1: Likewise.
* xgettext-glade-6: Likewise.
* xgettext-glade-7: Likewise.
* xgettext-gsettings-1: Likewise.
* xgettext-perl-2: Likewise.
* xgettext-perl-3: Likewise.
