Attempt to pacify pylint in bignum tests

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/scripts/mbedtls_dev/bignum_mod_raw.py b/scripts/mbedtls_dev/bignum_mod_raw.py
index 0084898..461b1f2 100644
--- a/scripts/mbedtls_dev/bignum_mod_raw.py
+++ b/scripts/mbedtls_dev/bignum_mod_raw.py
@@ -64,21 +64,11 @@
     input_style = "arch_split"
     arity = 1
     suffix = True
-
-    @property
-    def is_valid(self) -> bool:
-        return self.int_a > 0 and self.int_a < self.int_n
-
-    @property
-    def arg_a(self) -> str:
-        # Input has to be given in Montgomery form
-        mont_a = self.to_montgomery(self.int_a)
-        return self.format_arg('{:x}'.format(mont_a))
+    mongtomgery_form_a = True
+    disallow_zero_a = True
 
     def result(self) -> List[str]:
-        result = bignum_common.invmod(self.int_a, self.int_n)
-        if result < 0:
-            result += self.int_n
+        result = bignum_common.invmod_positive(self.int_a, self.int_n)
         mont_result = self.to_montgomery(result)
         return [self.format_result(mont_result)]