test: MPC + stream component test (part 1)

[jakmeier]

Create a MockStream that connects to the MPC fixture
setup. This lets us test the indexer stream <-> MPC glue.

This is part 1, with just the tests adjusted to pass with the current configuration.

See #739 for tests that show current issues that need fixing.

[jakmeier]

This is no longer blocked with #765 merged.

@volovyks I think you already reviewed most of this in the draft (#739).
This is the same but with test settings changed that they pass.

[jakmeier]

@volovyks sorry, test_sign_no_presignature_waste apparently is still flaky in CI, I will ping you again when it's resolved

[jakmeier]

just a quick update: it succeeds locally but fails in CI, probably it is the same problem as with test_channel_contention_multiple_blocks_at_once_delayed in #739. Round alignment problems. It worked in CI with ACCEPT_POSIT_TIMEOUT = 500ms but fails again with ACCEPT_POSIT_TIMEOUT = 100ms.

Basically, the current solution is still quite fragile. I need to look in to this more to decide if we can just increase the timeout for the tests, or if we can solve the underlying problem. (100ms for network delay should be plenty, even on slow CI machines)

[jakmeier]

in theory, this should be passing tests now... but I did a bad rebase (clean in git but causes rust conflicts)

The trait ChainStream was changed on develop. I will have to fix that.