QtBluetooth - device discovery test on OS X.

- Enable this auto test on OS X, requires a modification: 'stop' is synchronous
  and so cancelSpy.isEmpty() == false (the last QVERIFY fails.

- Fix several problems with SDK versions (workarounds for non-existing methods in SDK < 10.9).
- Change error handling: we set an error in a ctor _only_ if QBluetoothAddress parameter was
  not null (!isNull()) and we failed to find a local device with such an address. This is the
  only case documented/expected, all others _do_ _not_ set an error, even if a local adapter
  is off or does not exits. This fixes a failing (to integrate) auto-test.

Change-Id: I9902211078330f296775f1c3249da166c19001a4
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>