Update populate-depot.py

• tests: Don't rely on being able to put more than one suite in a depot

  The SteamLinuxRuntime scripts have phased out support for populating a depot directory with a mixture of scout and soldier, because that doesn't really make sense for a game or a compat tool: each native Linux game, and each compat tool used to run non-native games, expects to run in one particular environment (scout or soldier or something else) and using any other environment would be incorrect.

  However, tests/pressure-vessel/containers.py currently expects to work with a directory that contains both scout and soldier. For now, we'll download scout and soldier separately, then mash them together into the same directory.

• Update populate-depot.py from SteamLinuxRuntime v0.20210608.0

• tests: Unpack runtimes into non-versioned directories

  tests/pressure-vessel/containers.py currently expects this, but the default behaviour for populate-depot.py changed between v0.20210430.0 and v0.20210505.0.

• tests: Include archives as well as unpacked directories

  Since SteamLinuxRuntime v0.20210505.0, the default is to use unpacked runtimes or include the archives, but tests/pressure-vessel/containers.py wants both.

• containers test: Wrap long lines

• tests: Time how long is taken for various aspects of containers test

• tests: Always download both scout and soldier

  Now that we're invoking populate-depot.py once per runtime branch, it's straightforward to use different versions for different runtimes.