test-data: Allow tests to be run when Btrfs is not available.

Create the fedora-btrfs.img as an empty file.

The only place this is used explicitly is tests/mountable/
test-mountable-inspect.sh, but that test already skips if !btrfs.

Also this is used via guests-all-good.xml, but the script that creates
this XML skips the file if it has zero size.

test-data/phony-guests/make-fedora-img.pl

@@ -123,6 +123,23 @@ EOF
 }
 
 elsif ($ENV{LAYOUT} eq 'btrfs') {
+    # Test if btrfs is available.
+    my $g2 = Sys::Guestfs->new ();
+    $g2->add_drive ("/dev/null");
+    $g2->launch ();
+    my $btrfs_available = $g2->feature_available (["btrfs"]);
+    $g2->close ();
+
+    if (!$btrfs_available) {
+        # Btrfs not available, create an empty image.
+        push (@images, "fedora-btrfs.img");
+
+        unlink ("fedora-btrfs.img");
+        open (my $img, '>', "fedora-btrfs.img");
+        close ($img) or die;
+        exit 0;
+    }
+    else {
         push (@images, "fedora-btrfs.img-t");
 
         open (my $fstab, '>', "fedora.fstab") or die;
@@ -151,6 +168,7 @@ EOF
         $g->umount ('/');
 
         $g->mount ('btrfsvol:/dev/sda2/root', '/');
+    }
 }
 
 elsif ($ENV{LAYOUT} eq 'lvm-luks') {