pytest: Add throttler to limit load on the test system

Both my machine and apparently the CI tester machines regularly run
into issues with load on the system, causing timeouts (and
unresponsiveness). The throttler throttles the speed with which new
instances of c-lightning get started to avoid overloading. Since the
plugin used for parallelism when testing spawns multiple processes we
need to lock on the fs. Since we have that file open already, we'll
also write a couple of performance metics to it.

contrib/pyln-testing/pyln/testing/fixtures.py

@@ -1,6 +1,6 @@
 from concurrent import futures
 from pyln.testing.db import SqliteDbProvider, PostgresDbProvider
-from pyln.testing.utils import NodeFactory, BitcoinD, ElementsD, env, DEVELOPER, LightningNode, TEST_DEBUG
+from pyln.testing.utils import NodeFactory, BitcoinD, ElementsD, env, DEVELOPER, LightningNode, TEST_DEBUG, Throttler
 from typing import Dict
 
 import logging
@@ -198,7 +198,12 @@ def teardown_checks(request):
 
 
 @pytest.fixture
-def node_factory(request, directory, test_name, bitcoind, executor, db_provider, teardown_checks, node_cls):
+def throttler():
+    yield Throttler()
+
+
+@pytest.fixture
+def node_factory(request, directory, test_name, bitcoind, executor, db_provider, teardown_checks, node_cls, throttler):
     nf = NodeFactory(
         request,
         test_name,
@@ -206,7 +211,8 @@ def node_factory(request, directory, test_name, bitcoind, executor, db_provider,
         executor,
         directory=directory,
         db_provider=db_provider,
-        node_cls=node_cls
+        node_cls=node_cls,
+        throttler=throttler,
     )
 
     yield nf