LE L2CAP cert tests: don't repeat the address

Define both the DUT and CERT address in one place and use it through the
whole test.
Those addresses will be obtained from proper facade in future versions.

Test: gd/cert/run --host --test_filter=LeL2capTest
Bug: 152348535
Change-Id: I3908ed019eedc49095a8029a357501f722b67dad