[test] buffered I/O tests for TLS 1.3 and TLS 1.2 SSL read paths

Cover normal (non-error) data flow through the code paths changed by
the read_nonblock exception fix and the netReadData.position() guard:

- Multi-chunk read_nonblock: 30KB echo in 1KB chunks (TLS 1.3 + 1.2)
- Multi-chunk with exception:false (TLS 1.3)
- Partial read_nonblock: read 5 bytes then remainder from appReadData
- Multiple puts/gets cycles: 10 rounds on a single connection
- sysread/syswrite round-trip: 3 blocking cycles with exact byte counts
- Large server write: server sends 48KB, client reads in 4KB chunks;
  regression test for netReadData.position()==0 guard (TLS 1.3 + 1.2)

Author: kares