Fix missing constant OpenStruct in tests

[agrare]

Fixes

Failures:

  1) InventoryRefresh::Graph#to_graphviz prints the graph
     Failure/Error: let(:node1) { OpenStruct.new(:whatever => 'foo') }
     
     NameError:
       uninitialized constant OpenStruct
     
         let(:node1) { OpenStruct.new(:whatever => 'foo') }
              
  2) InventoryRefresh::Graph#to_graphviz prints the graph with layers
     Failure/Error: let(:node1) { OpenStruct.new(:whatever => 'foo') }
     
     NameError:
       uninitialized constant OpenStruct
     
         let(:node1) { OpenStruct.new(:whatever => 'foo') }
        
Finished in 13.96 seconds (files took 0.76564 seconds to load)
235 examples, 2 failures, 2 pending

Failed examples:

rspec ./spec/graph_spec.rb:22 # InventoryRefresh::Graph#to_graphviz prints the graph
rspec ./spec/graph_spec.rb:38 # InventoryRefresh::Graph#to_graphviz prints the graph with layers

https://github.com/ManageIQ/inventory_refresh/actions/runs/8856834572/job/24323579863?pr=130#step:6:139

[miq-bot]

Checked commit agrare@5d4efeb with ruby 2.7.8, rubocop 1.56.3, haml-lint 0.51.0, and yamllint
1 file checked, 0 offenses detected
Everything looks fine. ⭐

[Fryguy]

I saw this in another gem and I think some version of json was released that dropped an inner dependency on ostruct.