Improve tests for the REPLCompletion

I have improved the test coverage for the `REPLCompletion.jl` to cover all lines on windows except:
* https://github.com/dhoegh/julia/blob/deb70b4498639db7face4674ddede52c09b61420/base/REPLCompletions.jl#L43
* https://github.com/dhoegh/julia/blob/deb70b4498639db7face4674ddede52c09b61420/base/REPLCompletions.jl#L122

One improvement could be more Unicode testing.