Floss: Extend the size of socket send in btclient

Some tests require to test L2CAP fragmentation, which requires to send
data larger than minimum MTU (48 bytes). This is done by extending the
default socket send data to 72 bytes.

Bug: 275266923
Test: run L2CAP/COS/CFD/BV-09-C
Change-Id: I5e240e5a81d588046939675b7f7cd39a0cc8c242