Add a dedicated crate for testing ff/group implementors

Provides extensive testing for dalek-ff-group and ed448.

Also includes a fix for an observed bug in ed448.

Cargo.lock

@@ -1512,6 +1512,7 @@ dependencies = [
  "curve25519-dalek 3.2.0",
  "digest 0.10.6",
  "ff",
+ "ff-group-tests",
  "group",
  "rand_core 0.6.4",
  "subtle",
@@ -2331,6 +2332,15 @@ dependencies = [
  "subtle",
 ]
 
+[[package]]
+name = "ff-group-tests"
+version = "0.12.0"
+dependencies = [
+ "group",
+ "k256",
+ "p256",
+]
+
 [[package]]
 name = "fiat-crypto"
 version = "0.1.17"
@@ -4570,6 +4580,7 @@ dependencies = [
  "dalek-ff-group",
  "digest 0.10.6",
  "ff",
+ "ff-group-tests",
  "generic-array 0.14.6",
  "group",
  "hex",