tests: Initialize random group elements fully

Summary:
Also fix add a missing comment.

This is a backport of secp56k1 [[bitcoin-core/secp256k1#814 | [[bitcoin-core/secp256k1#814 | PR814]]]]

Test Plan:
  ninja check-secp256k1

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7613

src/group.h

@@ -59,6 +59,7 @@ static int secp256k1_ge_is_infinity(const secp256k1_ge *a);
 /** Check whether a group element is valid (i.e., on the curve). */
 static int secp256k1_ge_is_valid_var(const secp256k1_ge *a);
 
+/** Set r equal to the inverse of a (i.e., mirrored around the X axis) */
 static void secp256k1_ge_neg(secp256k1_ge *r, const secp256k1_ge *a);
 
 /** Set a group element equal to another which is given in jacobian coordinates */

src/tests.c

@@ -86,6 +86,7 @@ void random_group_element_test(secp256k1_ge *ge) {
             break;
         }
     } while(1);
+    ge->infinity = 0;
 }
 
 void random_group_element_jacobian_test(secp256k1_gej *gej, const secp256k1_ge *ge) {