test: Call qemu with -nodefaults

Benjamin Drungrequested to merge
bdrung/initramfs-tools:qemu-nodefaults into master

To potentially reduce the execution time a bit, call qemu with -nodefaults. Only the serial console is needed to be setup.

The execution time of amd64-virtio-only on an AMD Ryzen 7 5700G system reduced from 10.01 +- 0.04 seconds to 9.82 +- 0.04 seconds. This improvement is neglectable on amd64, but could be noticeable on slower architectures.

This pull request depends on the changes from !76 (merged).

Edited by Benjamin Drung

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