c# - Add additional search path to NUnit testing -

i have question how add addtional search path nunit testing. although know it's not best practice, need reason.

let me description directory structure first:

rootfolder     unit-test.dll     folder1         a.dll         folder1-1             b.dll     folder2         c.dll 

i have put unit test dll in rootfolder, , under there many folders , sub-folders. in each folder/sub folder there dlls referenced test dll.
when run test dll nunit, want test dll load reference dll in sub-folder when needed.
know work if put reference dll in root folder(same folder test dll) don't want break directory structure.