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/suites/test_suite_bignum.function b/tests/suites/test_suite_bignum.function
index c90f1bb..35900e6 100644
--- a/tests/suites/test_suite_bignum.function
+++ b/tests/suites/test_suite_bignum.function
@@ -24,7 +24,7 @@
      * we sometimes test the robustness of library functions when given
      * a negative zero input. If a test case has a negative zero as input,
      * we don't mind if the function has a negative zero output. */
-    if (!mbedtls_test_case_uses_negative_0 &&
+    if (!mbedtls_test_get_case_uses_negative_0() &&
         mbedtls_mpi_bitlen(X) == 0 && X->s != 1) {
         return 0;
     }