This change alters `ExercismGenerator` just enough to allow side effect free
tests. Having the class under test enables future refactoring. Three dependancies
made this class difficult to test:
- `PipeableOSProcess` executed commands outside of the Pharo VM
- `ExTonelWriter` wrote files to the OS filesystem
- `ExercismExercise` would access a variable number of exercises depending on how many were implemented

To get this class under test the above dependancies have been placed in lazy
assigned instance variables. This allows tests to assign mocks to the generator
instance.

There are two mock OS processes, one that always succeeds, and one that always fails.
Neither executes any function outside the VM.

A  `ExTonelWriter` writes exercises to an in memory file system that lasts only
for the test.

A mock exercism exercise class returns only a single exercise. This speeds up the
test and makes it more consistent as it won't change when more exercises are implemented.

The change is a test that passed on my personal computer (MacOS) but failed in CI (Ubuntu 16.04).

I suspect that even though the tests use an in memory file system OS specific line-feed and carrage-return
characters will be used causing enough of a difference between the expected result and the actual
result of the test to fail. This change makes the line ending characters of the actual and expected results
identical before asserting.

The test design closely follows that of `TonelWriterTest` which also uses an in memory file system.