Remove the following steps, since they are rarely (if never) used and
only increase the complexity of the `build.zig` file:
- install step (zig build -Dn=n install)
- test step (zig build -Dn=n test)
- start step (zig build -Dn=n start)
The only remaining step is the default zigling step (zig build -Dn=n),
where the user can choose the exercise to solve.
Update the tests. Additionally, update CheckNamedStep and CheckStep so
that they can share the same functions.
Closes#299
dc2539ec4e