Clean up and fix `guest_generate_testdata.py` #64

Job	Run time
static / formatcheck	31s
tests / makefile	1m 8s
static / cppcheck	16s
tests / cmake	51s
tests / makefile (GUEST_BACKEND=0)	1m 25s
tests / cmake (-DGUEST_BACKEND=0)	1m 16s
tests / makefile (HOST_BACKEND=0)	26s
tests / cmake (-DHOST_BACKEND=0)	1m 10s
	7m 3s

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