tests: add TEST_QEMU_ONLY variable to run only tests where qemu is mandatory

Allows to split the test run in two parts. Most tests can run under
nspawn which is much faster, and they can be ran in one chunk with
TEST_NO_QEMU=1. The qemu-only tests, which are just a handful, can
be ran in another chunk with TEST_QEMU_ONLY=1.

Allows autopkgtest to be split in two parts.