Tests: bluetooth: controller: add correct number of max connections

The number of connections for the tx_buffer_alloc needs to be at least 3
For one of the tests it was set to default 1, which leads to unspecified
behaviour.

This is fixed by setting the maximum nr. of connections to 4

Signed-off-by: Andries Kruithof <andries.kruithof@nordicsemi.no>