eth, catalyst: fix flaky tests

[holiman]

This is another PR to fix flaky tests, mostly due to built-in synchronicity requirements which do not work very well on the low-resource appveyor machines.

Fix 1:

--- FAIL: TestNewPayloadOnInvalidChain (0.14s)
    api_test.go:604: payload should not be empty

Fix 2:

--- FAIL: TestTransactionPropagation67 (1.76s)
    handler_eth_test.go:455: sink 0: transaction propagation timed out: have 0, want 1024
INFO [11-10|03:48:40.490] Writing custom genesis block