Move bignum flag for negative zero into test_info

Add accessors ready for protection with test_info mutex.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/tests/src/bignum_helpers.c b/tests/src/bignum_helpers.c
index c85e2ca..913f5e3 100644
--- a/tests/src/bignum_helpers.c
+++ b/tests/src/bignum_helpers.c
@@ -135,7 +135,7 @@
     }
     if (negative) {
         if (mbedtls_mpi_cmp_int(X, 0) == 0) {
-            ++mbedtls_test_case_uses_negative_0;
+            mbedtls_test_increment_case_uses_negative_0();
         }
         X->s = -1;
     }