bluetooth: tester: Use fixed value for BT_BUF_ACL_RX_SIZE

Default BT_BUF_ACL_RX_SIZE value varies based on selected host options.
Many buffers size (L2CAP COC MPS in particular) depends on that value
and those may affect qualification test cases steps.

To make core tests more predictable just use fixed values big enough to
comply with all configurations requirements.

This was affecting L2CAP/COS/ECFC/BV-04-C.

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>