tor-netdoc: b64 tests: add exhaustive roundtrip test
!608 (merged) revealed that my adhoc hand-c&p test cases for b64 decoding had a mistake. Here I make another effort to spot any further similar bugs. I tried these tests with the pre-!608 code and they fail there, as hoped.
CC @arturomf94 who has been doing most of actual the work in this area (thanks!)