Test lastindex for LazyString (#56585)

Should get the lazy strings file to 100% coverage

Author: kshyatt

test/strings/basic.jl

@@ -1195,6 +1195,7 @@ end
     @test codeunit(l) == UInt8
     @test codeunit(l,2) == 0x2b
     @test isvalid(l, 1)
+    @test lastindex(l) == lastindex("1+2")
     @test Base.infer_effects((Any,)) do a
         throw(lazy"a is $a")
     end |> Core.Compiler.is_foldable