mkosi: update mkosi.fedora so that we can run "make check" during build

Our tests require "diff" and "tree", hence let's add them to the set of
development packages.

.mkosi/mkosi.fedora

@@ -27,7 +27,7 @@ Format=raw_btrfs
 Bootable=yes
 
 [Partitions]
-RootSize=2G
+RootSize=3G
 
 [Packages]
 BuildPackages=
@@ -37,6 +37,7 @@ BuildPackages=
         bzip2-devel
         cryptsetup-devel
         dbus-devel
+        diffutils
         docbook-style-xsl
         elfutils-devel
         gcc
@@ -69,4 +70,5 @@ BuildPackages=
         python3-devel
         python3-lxml
         qrencode-devel
+        tree
         xz-devel