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

By whitelisting the peers via -whitelist, the inventory is transmissioned
immediately rather than on average every 5 seconds, speeding up the test by at
least a factor of two:

before:
$ time ./interface_rest.py
...
0m14.82s real     0m01.44s user     0m01.19s system

with this commit:
$ time ./interface_rest.py
...
0m05.67s real     0m01.07s user     0m01.35s system

Author: theStack

test/functional/interface_rest.py

@@ -52,6 +52,9 @@ class RESTTest (BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 2
         self.extra_args = [["-rest", "-blockfilterindex=1"], []]
+        # whitelist peers to speed up tx relay / mempool sync
+        for args in self.extra_args:
+            args.append("[email protected]")
         self.supports_cli = False
 
     def test_rest_request(self, uri, http_method='GET', req_type=ReqType.JSON, body='', status=200, ret_type=RetType.JSON):