Docker-Compose Environment-Variables blank string

Although the other answers are both correct they do not highlight the underlying misunderstanding here enough:

With the env_file option you can specify a file with variables to be injected into the environment in the container.

Using variable substitution in the docker-compose.yml you can access variables in the environment of the docker-compose command, i.e. on the host.
You can set these using the usual mechanisms of your OS/shell, e.g. in bash:

export XUSER=you
docker-compose up

Additionally with docker-compose you can use a .env file in the current directory.

So in your concrete example you should just move env/test.env to .env to add the variables to the environment of docker-compose for variable substitution.
If you also want to add them to the environment in the container you can do it like this:

version: '3'
services:
  abc:
    build: .
    image: xyz:latest
    container_name: xyz

    # add variables from the docker-compose environment to the container:
    environment:
      - XUSER=$XUSER
      # or even shorter:
      - XHOME

    # use variable from the docker-compose environment in the config:
    user: "${XUSER}"