Replace TEST_ASSERT("message" == 0) with TEST_FAIL

Signed-off-by: Agathiyan Bragadeesh <agathiyan.bragadeesh2@arm.com>
diff --git a/tests/suites/test_suite_bignum.function b/tests/suites/test_suite_bignum.function
index 7f858e5..7ccc481 100644
--- a/tests/suites/test_suite_bignum.function
+++ b/tests/suites/test_suite_bignum.function
@@ -834,7 +834,7 @@
     } else if (strcmp(result_comparison, "!=") == 0) {
         TEST_ASSERT(mbedtls_mpi_cmp_mpi(&Z, &A) != 0);
     } else {
-        TEST_ASSERT("unknown operator" == 0);
+        TEST_FAIL("unknown operator");
     }
 
 exit: