Raise tortls unit test coverage as high as possible
Our TLS wrappers really ought to be completely covered by unit tests of one kind or another. Right now, they're among the worst-covered functions in src/common:
x/tortls.c.gcov 593 355 37.45
TOTAL 593 355 37.45
It might also be a good idea to split up these functions into backend wrappers vs functions based on those wrappers for how tor uses TLS