test(string-literal): fix mermaid diagrams comparison

The intent of this test was to ensure rendering a Mermaid diagram is
equivalent when using the `mermaid` string literal or the `Diagram` type
directly. What it was testing was whether the resulting `Diagram`
instances were equivalent.

With the introduction of the `options` field in
1c814b8, this is no longer the case as
a `Dict{String,String}()` does not equal another
`Dict{String,String}()`. This caused the test to fail. What _should be
compared is the rendered diagram.

test/kroki/string_literals_test.jl

@@ -18,7 +18,7 @@ using Kroki: Diagram, @mermaid_str, @plantuml_str
 
   string_literal_mermaid = mermaid"graph TD; A --> B"
   diagram_type_mermaid = Diagram(:mermaid, "graph TD; A --> B")
-  @test string_literal_mermaid == diagram_type_mermaid
+  @test "$string_literal_mermaid" == "$diagram_type_mermaid"
 
   @testset "support interpolation" begin
     # String macros do no support string interpolation out-of-the-box, this