tests: separate-store-os: Increase root partition size.

* gnu/tests/install.scm (%separate-store-installation-script): Increase
size of the root partition so that it's big enough to contain
downloaded nars and temporary files.  Add 'df -h /mnt/gnu'.

gnu/tests/install.scm

@@ -504,8 +504,8 @@ export GUIX_BUILD_OPTIONS=--no-grafts
 guix build isc-dhcp
 parted --script /dev/vdb mklabel gpt \\
   mkpart primary ext2 1M 3M \\
-  mkpart primary ext2 3M 100M \\
-  mkpart primary ext2 100M 1.2G \\
+  mkpart primary ext2 3M 400M \\
+  mkpart primary ext2 400M 2.1G \\
   set 1 boot on \\
   set 1 bios_grub on
 mkfs.ext4 -L root-fs /dev/vdb2
@@ -514,6 +514,7 @@ mount /dev/vdb2 /mnt
 mkdir /mnt/gnu
 mount /dev/vdb3 /mnt/gnu
 df -h /mnt
+df -h /mnt/gnu
 herd start cow-store /mnt
 mkdir /mnt/etc
 cp /etc/target-config.scm /mnt/etc/config.scm