secureConnecting is never set to false on client TLS sockets.
So if Http2Session constructor (in lib/internal/http2/core.js) is
called after secureConnect is emitted, then it will wrongly wait
for a secureConnect event.

This fix sets secureConnecting to false when a client TLS socket
has connected.

PR-URL: nodejs#33209
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>

Without this, the session is destroyed with the following error

```
Error [ERR_HTTP2_ERROR]: Protocol error
    at Http2Session.onSessionInternalError (internal/http2/core.js:756:26)
Emitted 'error' event on ClientHttp2Session instance at:
    at emitClose (internal/http2/core.js:1010:10)
    at internal/http2/core.js:1048:7
    at finish (internal/streams/writable.js:731:5)
    at processTicksAndRejections (internal/process/task_queues.js:80:21) {
  code: 'ERR_HTTP2_ERROR',
  errno: -505
}
```

The test then calls `session.close()` which tries to write to a
destroyed socket. As a result, an unhandled `ECONNRESET` error is
emitted in the v12 release line.

PR-URL: nodejs#35482
Refs: nodejs#34859
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>