fix(primitive_types4): Fail on a slice covering the wrong area

This commit converts primitive_types4 to a test and asserts that the
slice given is equal to the expected slice.

The intent of the primitive_types4 exercise appears to be to ensure the
user understands inclusive and exclusive bounds as well as slice syntax.
`rustlings` commands using `compile` do not verify that a specific
println is reached and, in the case of `watch` and `verify` (but not
`run`), they do not output the `println`s at all.

This fix is semantically similar to #198. It does not take a stance on
the correct way to handle this for all exercises; see #127. There are
likely other exercises whose intent are masked by this issue.

info.toml

@@ -66,7 +66,7 @@ mode = "compile"
 
 [[exercises]]
 path = "exercises/primitive_types/primitive_types4.rs"
-mode = "compile"
+mode = "test"
 
 [[exercises]]
 path = "exercises/primitive_types/primitive_types5.rs"