Add a 5 minute timeout to python tests.

The CI times out after 10 minutes of no output. It's annoying if a
test hangs and is killed by the CI timeout, because you don't get
information about which test was running. Try to avoid that, by adding
a slightly smaller timeout in pytest itself. You can override it on a
per-test basis if needed, but let's try to keep our tests shorter than
that.

For the Postgres regression tests, use a longer 30 minute timeout.
They're not really a single test, but many tests wrapped in a single
pytest test. It's OK for them to run longer in aggregate, each
Postgres test is still fairly short.

test_runner/performance/test_startup.py

@@ -1,9 +1,11 @@
+import pytest
 from contextlib import closing
-
 from fixtures.zenith_fixtures import ZenithEnvBuilder
 from fixtures.benchmark_fixture import ZenithBenchmarker
 
 
+# This test sometimes runs for longer than the global 5 minute timeout.
+@pytest.mark.timeout(600)
 def test_startup(zenith_env_builder: ZenithEnvBuilder, zenbenchmark: ZenithBenchmarker):
     zenith_env_builder.num_safekeepers = 3
     env = zenith_env_builder.init_start()