L2cap: Fix ERTM test Send S-Frame [REJ]

Under some circunstances, the stack might enter "local busy" state, and
sending an "Receiver Not Ready" packet. Almost immediately the stack
exits "local busy" state and sends "Receiver Ready" packet with P=1.
We should allow this situation in cert test, and not explicitly require
P=0.

We still need to figure out why we enter and exit "local busy" state.
Currently it solely depends on queue size on client queue end, and it
detects local busy when there is more than 1 pending item. Maybe we can
set the threshold to 2 if this occurs often and causes real problem.

Test: cert/run --host
Tag: #gd-refactor
Bug: 157251860
Change-Id: I33cbd46d266865f396edb624550b8c7cb38f51a8