Add a smoke-test for the SteamLinuxRuntime

• .gitignore: Ignore some generated/downloaded/downloadable files

  These files need to be downloaded or otherwise filled in during the process of building a complete depot.

• Add a smoke-test for the SteamLinuxRuntime

  See the new tests/README.md and tests/depot/README.md files for details.

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https://phabricator.collabora.com/T17570

Tested in https://jenkins.internal.steamos.cloud/job/playground/job/scout/job/build/134/.

/cc @jpwhiting @denittis @timo

Initially assigning to @jpwhiting for review since it's related to dev-tools!23, but review from others would be equally welcome.