samples: drivers: mbox: fix nRF54H20 vevif/bellboard regexes

Both VEVIF and BELLBOARD test cases are unidirectional from the core
executing the test (cpuapp). For VEVIF, cpuapp pings, and for BELLBOARD,
cpuapp pongs.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>