Stabilize tst_qwebenginedownloads

The waitForRequest/waitForSignal function used by these tests is broken: it
assumes that calling QEventLoop::exit() will immediately exit from the event
loop. In actuality this is not immediate and the event loop may continue to
execute more signals handlers even after exit() has been called. This means that
in e.g. waitForRequest the 'result' variable may be assigned to twice.

Additionally there is a race condition in downloadTwoLinks, where we sometimes
skip the first download. This is not a bug but simply one pending navigation
being aborted in favor of another.

Changes

  - Delete waitForRequest. Define one HTTP request handler per test, using state
    variables to communicate with the main body of the test. Ignore unknown
    requests (including favicon requests). Same for downloadRequested signals.

  - Expand downloadTwoLinks and fix expectations.

  - Add logging to HTTPServer.

  - Unblacklist.

Task-number: QTBUG-66888
Change-Id: I718cac6c4b32a8cc68400fa8ee5b853686c77fcb
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>