provide CLI option '-d' for setting TEST_DIR

[sbulka]

Unfortunately I'm not into .bat (yet), and have no windows machine in reach atm.

If accepted, should definitely add it to xspec.bat too.

[sbulka]

need to add tests for option -d.
need to edit tests for all help output.

[AirQuick]

Just curious. What is your use case?
Note TEST_DIR can be set externally.

[sbulka]

Mainly comfort.

If it is only the variable, I wouldn't want it in the help output.
As an option, it would be mentioned on the cmd line help, which is easier than looking up documentation in wiki.
To make my colleagues adapt xspec too, I need it as convenient as possible.

I frequently use the unzip option -d, so it feels natural for me to expect something like this here too.