Speed up tail.test for shells without builtin printf.

Speed up tail.test for shells without builtin printf.

The test only needs echo, and Android currently doesn't use mksh's
builtin printf; the toybox printf is fuller featured and less buggy,
but a lot slower to call in a loop like this (and mksh doesn't fall
back to the printf on the path if given something it can't handle,
so we can't have the best of both worlds).

Without this hack, the tail test takes as long as all the other
tests put together.