The point of this function is to make sure that the ed25519-based
implementation of curve25519_basepoint() actually works when we
start tor, and use the regular fallback implementation if it
doesn't.  But it accounts for 9% of our startup time in the case
when we have directory information, and I think it's safe to make
the test shorter.  After all, it has yet to find any actual bugs in
curved25519_scalarmult_basepoint_donna() on any platforms.

Closes ticket 28838.