this causes all libfaketime tests to fail, due to unset
environment variable (which is set in init). Previously
this was no problem because the environment variable did
not exist in any case. Thus the problem went undetected
for a long time.
This hides the actual location of the config file from the
test, which is better in many aspects (I guess at least
"make distcheck" would otherwise kick our ass at some time...).
