Deflake and clean up lazy-loading tests (#1146)
Deflake and clean up lazy-loading tests (#1146)

* Deflake `Old leaves are present in gapped incremental syncs`

Hopefully, deflake this, by making sure that all of Bob's events hit the
synchrotron before trying to sync from Alice.

Fixes #1037

* Deflake `Gapped incremental syncs include all state changes`

Give this one the same treatment.

Fixes #1043.

* Deflake `Leaves are present in non-gapped incremental syncs`

Another case of the same.

Fixes #1045

* Update `The only membership state in initial sync`...

This is non-functional, just a cleanup:

 * use `send_filler_messags_synced` instead of a loop
 * use `check_filler_event` instead of a local definition
 * fix a warning `Use of uninitialized value in numeric eq (==)`
 * use `die` instead of `croak`, given we want to log the right line number.

* Use `matrix_send_filler_messages_synced` elsewhere

... for consistency, and because it's shorter.
matrix-org/sytest,matrix-org/sytest