tests: subsys: zbus: do not use verbose logging

This change fixes a regression on `qemu_riscv64_smp` and
`qemu_riscv32_smp` that arose because debug logging was
enabled. My guess is that subtle races exist and that
debug logging exacerbates the condidtions for those
races.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>