pytest: Performance improvements and test stabilization

[cdecker]

This is intended mostly as a tracking issue to see if we can speed up pytest (the tests, not the framework) and stabilize some flaky tests.

Performance

Running the pytest target with PYTEST_OPTS=--profile-svg yields the following profile on my machine:

Notice that the profile accounts wall clock time, not CPU usage.

Slowest tests

Notice that there still might be a good reason for these to be slow, I'm just listing the slowest 5:

• [ ] test_pay_disconnect_stress 15%
• [ ] test_partial_payment_timeout 12%
• [ ] test_feerate_stress 9%
• [ ] test_plugin_shutdown 9%
• [ ] test_partial_payment 7.5%

Flaky

List of flaky tests:

• [ ]