tests: subsys: logging: log_msg2: disable some tests on qemu_cortex_a9

logging.log_msg2_64b_timestamp and logging.log_msg2_fp_64b_timestamp are
currently failing on qemu_cortex_a9 due to #39978. Disable them on that
platform until the issue is fixed.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>