Enforce fees in the integration tests so that
tests can be written to cover the fee rejection
logic. This was previously unit tested in geth
without end to end test coverage. Projects
depend on the error strings returned, so it is
important to have test coverage over them.

Previously the owner of the gas price oracle was set as
the same wallet used in the `l2Wallet` in the
integration tests. This would allow it to send
0 gas price transactions, as the sequencer will not
charge for transactions from the owner of the gpo.
The sequencer also updates the cached values
from the contract whenever the owner of the gpo
sends a transaction, which results in extra
logs and makes the test environment less realistic.