Add some small additional tests

[lums-rai]

First attempt at adding some very small tests to ReLint.jl to catch some simple things in the style guide that are not currently being caught.

In particular, these rules are added:

struct BareUsingRule <: ViolationLintRule end
struct UntypedArrayComprehensionRule <: ViolationLintRule end
struct ReturnTypeAnnotationRule <: RecommendationLintRule end
struct StringConcatenationRule <: RecommendationLintRule end
struct NoGlobalVariablesRule <: RecommendationLintRule end

Full disclosure, I used claude to help write these. But everything seems to be correct -- there are tests, which seem reasonable, and which pass.

[bergel]

That is an excellent contribution! Thank you @lums658