Decide the fate of qasn1element auto-test

qasn1element was never a public API and never had to be one. The reason we have it - in some backends (like Schannel or SecureTransport, and previously WinRT) some API was missing and thus this code exists. When moving backends into plugins, we (I mean, me) had no reason to keep this class inside QtNetwork - it's a low-level detail and workaround, would be quite an awkward and handicapped piece of public API to have. But moving it into 'shared' or 'generic' plugin code also means it cannot be tested as a part of qtbase/tests/auto/network/ssl. Testing it is still important (as bugs found in the past show).