test to show current behavior is wrong at parsing an environment file
defining an undefined variable - it must not be defined!
NOTE: this test assume the $HOME variable is always set (see POSIX, this
normally is the case, e.g. the test suite remains stable).
Signed-off-by: Tom Klingenberg <tklingenberg@lastflood.net>
`--label-file` has the exact same behavior as `--env-file`, meaning any
placeholder (i.e. a simple key, no `=` sign, no value), it will get the
value from the environment variable.
For `--label-file` it should just add an empty label.
Signed-off-by: Vincent Demeester <vincent@sbr.pm>