Merge pull request #2625 from tnull/2023-09-moar-tests-n-fixes

Improve test coverage of #2575 router fixes

diff --cc lightning/src/ln/outbound_payment.rs
Simple merge

diff --cc lightning/src/ln/payment_tests.rs
Simple merge

diff --cc lightning/src/routing/router.rs
Simple merge

diff --cc lightning/src/util/test_utils.rs
Simple merge