Here are instructions for testing fuse using the passthrough_ll example
filesystem provided in the libfuse source tree:

git clone git://github.com/libfuse/libfuse.git
cd libfuse
meson build
cd build
ninja
cat << EOF | sudo tee /sbin/mount.fuse.passthrough_ll
#!/bin/bash
ulimit -n 1048576
exec $(pwd)/example/passthrough_ll -ofsname="\$@"
EOF
sudo chmod +x /sbin/mount.fuse.passthrough_ll
mkdir -p /mnt/test /mnt/scratch /home/test/test /home/test/scratch

Use the following local.config file:

export TEST_DEV=non1
export TEST_DIR=/mnt/test
export SCRATCH_DEV=non2
export SCRATCH_MNT=/mnt/scratch
export FSTYP=fuse
export FUSE_SUBTYP=.passthrough_ll
export MOUNT_OPTIONS="-osource=/home/test/scratch,allow_other,default_permissions"
export TEST_FS_MOUNT_OPTS="-osource=/home/test/test,allow_other,default_permissions"