tests/integration: Use a minimal custom base sysroot

We have been using the Freedesktop SDK binaries to provide a base
system to run the integration tests. This works OK but it weighs in
at 985MB of content, and there is no simple way to customize it so
that we only download the bits we actually need.

This commit changes the tests to use a custom sysroot based on the
Alpine Linux distribution. The sysroot is 155MB unpacked, and packs down
to a 27MB .tar.xz.

This speeds up the integration tests significantly as we greatly reduce
the amount of network traffic required and the amount of data that gets
copied around when creating the staging area.