os_release: add i3 variant pair

Since we switched to building i3 with Kiwi (I think), it's being
properly identified, so we need to add it here for the test to
pass.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

tests/os_release.pm

@@ -103,6 +103,7 @@ sub run {
         KDE => ["kde", "KDE Plasma Desktop Edition"],
         Silverblue => ["silverblue", "Silverblue"],
         IoT => ["iot", "IoT Edition"],
+        i3 => ["i3", "i3"],
     );
     if (exists($variants{$subvariant})) {
         ($variant_id, $variant) = @{$variants{$subvariant}};