Try TOP even on POP servers that fail to report CAPA

* Ignore the results of CAPA and always try TOP
* If TOP returns -ERR simply fall back to (bad old slow) RETR
* Unit tests for positive & failure cases

Bug: 2588432
Change-Id: Ife4b551217de1025e14efc46074f16ef4ae99c6f
2 files changed