Following the implementation in `std.Build.Step.Compile, add the Kind
type to differentiate between a normal executable and a test executable
running zig tests. Replace `Exercise.run_test` field with `kind`.
Compile the exercise in both the exe and test cases, reducing code
duplication.
Add the `check_output` and `check_test` methods in ZiglingStep, in order
to differentiate the code checking a normal executable and a test
executable.
Update the tests to correctly check both the exe and test cases. Remove
the temporary code added in commit 832772c.
9ab9ebf33f