test/filesystem: move blueprint writing to function

Move the creation of the blueprint to a function.  We'll then add a
separate blueprint function for the new, alternative partitioning
customization.

test/cases/filesystem.sh

@@ -120,8 +120,9 @@ check_result () {
 
 greenprint "🚀 Checking custom filesystems (success case)"
 
-# Write a basic blueprint for our image.
-tee "$BLUEPRINT_FILE" > /dev/null << EOF
+write_fs_blueprint() {
+    # Write a basic blueprint for our image.
+    tee "$BLUEPRINT_FILE" > /dev/null << EOF
 name = "custom-filesystem"
 description = "A base system with custom mountpoints"
 version = "0.0.1"
@@ -195,6 +196,9 @@ mountpoint = "/foobar"
 size = 131072000
 EOF
 fi
+}
+
+write_fs_blueprint
 
 build_image "$BLUEPRINT_FILE" custom-filesystem qcow2 false