Use new backpack version in tests

[chapulina]

A new version has been uploaded for this model, and this is causing test failures.

This is the quick fix. The best fix would be to rely on a "test" model which we know won't be changed.

[codecov]

Codecov Report

Merging #455 (e016dcb) into ign-gazebo2 (59babe6) will increase coverage by 24.16%.
The diff coverage is 76.03%.

@@               Coverage Diff                @@
##           ign-gazebo2     #455       +/-   ##
================================================
+ Coverage        53.78%   77.94%   +24.16%     
================================================
  Files              121      187       +66     
  Lines             5838    10208     +4370     
================================================
+ Hits              3140     7957     +4817     
+ Misses            2698     2251      -447     

Impacted Files	Coverage Δ
include/ignition/gazebo/EntityComponentManager.hh	100.00% <ø> (ø)
include/ignition/gazebo/Server.hh	100.00% <ø> (ø)
include/ignition/gazebo/ServerConfig.hh	100.00% <ø> (ø)
...nclude/ignition/gazebo/components/Serialization.hh	61.29% <0.00%> (+61.29%)	⬆️
src/SimulationRunner.cc	95.19% <ø> (+24.44%)	⬆️
.../plugins/component_inspector/ComponentInspector.cc	9.41% <0.00%> (ø)
src/gui/plugins/entity_tree/EntityTree.cc	10.52% <ø> (ø)
src/gui/plugins/modules/EntityContextMenu.cc	19.14% <ø> (ø)
src/gui/plugins/shapes/Shapes.cc	31.25% <ø> (ø)
.../gui/plugins/transform_control/TransformControl.cc	33.33% <ø> (ø)
... and 153 more

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