symm/poly1305.c, etc.: Convert the Monte-Carlo tests to incremental form.

Similarly to the way 1e4c26653e52aa4c4b06e345617135a6ff271ab5 modified
the X25519 and X448 tests, arrange that each Monte-Carlo test vector
continues from where the previous one left off.  This is considerably
trickier to pull off because there's so much more state to maintain in
these tests, and changes to the processing code was necessary to make it
work.

Again, as in 1e4c2665..., move the very slow high-iteration-count tests
into a separate file, and arrange for it to be distributed.

Note that the outputs are unchanged by all of this.