Replace direct calls of toHtml and toPlainText to use a blocking
helper function that spins a QEventLoop to wait for the async result.
This should work fine for tests where the event loop is less polluted
by other events that could cause code reentrancy through stacked stacks.

Change-Id: Ic46a06a9abad782a39a620ceecdc51c3bbb6b5a1
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>