test: speedup p2p_feefilter.py by whitelisting peers (immediate tx relay)

Most of the test time is spent in wait_for_invs() after sending to addresses,
i.e. the bottleneck is in relaying transactions. By whitelisting the peers via
-whitelist, the inventory is transmissioned immediately rather than on average
every 5 seconds, speeding up the test significantly:

before:
$ time ./p2p_feefilter.py
...
real    0m39.367s
user    0m1.227s
sys 0m0.571s

with this commit:
$ time ./p2p_feefilter.py
...
real    0m9.386s
user    0m1.120s
sys 0m0.577s

0 files changed, 0 insertions, 0 deletions