How to modify process environment variables

[Ky-Anh Huynh via Digitalmars-d-learn]

Hi,

Is it possible to change the current process's environment 
variables?

I have looked at `std/process.d` source code, and there is only a 
private method `createEnv` used when new (sub)process is created.

In C `putEnv` the answer is positive: 
http://man7.org/linux/man-pages/man3/putenv.3.html (FIXME)

I come to this question as I want to set some custom variables 
for my unittests. My program reads some tokens from system 
environments, and it's convenient if I can simulate different 
cases for testings.

Thanks for your reading and support.