Add BeforeSuite file shiv.

[dkinzer]

REF CIVIC-6610

When stage file proxy is enabled we need to make sure some required files are present in public://
By adding a BeforeSuite step that copies these files from a site to the local
environment, we fix errors that arise from files not being present in the
testing environment.

QA Steps

On a local version of devy with this patch

• Enable the stage_file_proxy module
• Verify that config.yml default:stage_file_proxy_origin is set to devy prod or set.
• Verify that configy.yml default:stage_file_proxy_files points to default_log.png or set it
• Remove default_log.png from public:// files directory
• Remove all files like dkan-topic.* from public:// files directory
• Verify that the following test does not fail: dkan/test/features/theme.admin.feature:16
• Verify that the following test does not fail: dkan
• dkan/test/features/topics.feature:48

[dafeder]

This was re-rolled in #2018