aead_test: make AEAD selection table driven.

(The huge if-else was hard to visually parse.)

Change-Id: Ic2c94120f345085b619304181e861f662a931a29
Reviewed-on: https://boringssl-review.googlesource.com/4691
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>