Use the cmake ExternalData to download the test data

[jmcarcell]

BEGINRELEASENOTES

• Use ExternalData to download the test data at build time. See Use the cmake ExternalData module to manage test data AIDASoft/podio#508

ENDRELEASENOTES
In this case this is a bit different from the podio pull request because for people that are building only but not running the tests they will be downloading the files unnecessarily. This can be solved by passing a path to keep a local cache of the files when running cmake with -DExternalData_OBJECT_STORES=/path/to/folder.

[tmadlener]

Can you elaborate why in this case the files are downloaded unconditionally? I.e. what is different wrt the podio PR?

[jmcarcell]

It's the same but for this repo the files were only downloading when running the tests so for someone who builds them but doesn't run them this actually makes it worse by having to download them, although possibly only once if using ExternalData_OBJECT_STORES

[tmadlener]

Ah right, so the behavior of this changes slightly. I was looking for differences wrt podio.