Test suite setup doc: fix APT pinning on Buster systems
950fa6ae introduced usage of pinning by source package ("Package: src:") for Buster systems, but that syntax was introduced in APT 1.9.11, while Buster only has 1.8.x.
Note: this APT pinning record is a no-op in most practical cases: our QEMU 4.2 backport has a higher version than what's in Buster so it should be the preferred candidate. Still, in some cases this pinning is needed, so let's get it right.