`MatrixClient.setAccountData`: await remote echo. (#4695)
* Rewrite `deleteAccountData` test
use fetch-mock rather than whatever this was
* `MatrixClient.setAccountData`: await remote echo
Wait for the echo to come back from the server before we assume the account
data has been successfully set
* Update integration tests
Fix up the integ tests which call `setAccountData` and now need a sync
response.
* Address review comment
