with_env now overrides the environment, similar to with_stdin
an additional function update_env was created to mirror the prior
functionality of with_env, updating the env
This was changed as previously it was impossible to delete variables
from the env
replaced the code of .ok() with a call to .expect, to remove the code
duplication
Change-Id: I83933893c7f2ccfdc7bd4933b7592b475c435e76
Currently the Command and CommandResult classes are mixed into the nix
fixture file.
This commit moves them out into their own lib file, to make it more
obvious that they can be used standalone for other applications too
Additionally improved documentation of said classes
and bumped log level of stdout and err on unexpected exitcodes, as it is
within an error context
Change-Id: If2d554acde86fd54f2445fc46453f06923af5fe9
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