test: use btrfs' mkswapfile on btrfs

So it's created automagically with proper attributes.

test/TEST-55-OOMD/test.sh

@@ -15,9 +15,7 @@ test_append_files() {
     # Create a swap file
     (
         image_install mkswap swapon swapoff stress
-
-        dd if=/dev/zero of="${initdir:?}/swapfile" bs=1M count=48
-        chmod 0600 "${initdir:?}/swapfile"
+        image_install -o btrfs
 
         mkdir -p "${initdir:?}/etc/systemd/system/init.scope.d/"
         cat >>"${initdir:?}/etc/systemd/system/init.scope.d/test-55-oomd.conf" <<EOF

test/units/testsuite-55.sh

@@ -23,7 +23,14 @@ rm -rf /run/systemd/system/testsuite-55-testbloat.service.d
 
 # Activate swap file if we are in a VM
 if systemd-detect-virt --vm --quiet; then
-    mkswap /swapfile
+    if [[ "$(findmnt -n -o FSTYPE /)" == btrfs ]]; then
+        btrfs filesystem mkswapfile -s 64M /swapfile
+    else
+        dd if=/dev/zero of=/swapfile bs=1M count=64
+        chmod 0600 /swapfile
+        mkswap /swapfile
+    fi
+
     swapon /swapfile
     swapon --show
 fi