Separate out to_montgomery and from_montgomery for 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 0bbad5d..d05479a 100644
--- a/scripts/mbedtls_dev/bignum_mod_raw.py
+++ b/scripts/mbedtls_dev/bignum_mod_raw.py
@@ -92,10 +92,9 @@
     arity = 1
 
     def result(self) -> List[str]:
-        result = (self.int_a * self.r) % self.int_n
+        result = self.to_montgomery(self.int_a)
         return [self.format_result(result)]
 
-
 class BignumModRawConvertFromMont(bignum_common.ModOperationCommon,
                                   BignumModRawTarget):
     """ Test cases for mpi_mod_raw_from_mont_rep(). """
@@ -106,7 +105,7 @@
     arity = 1
 
     def result(self) -> List[str]:
-        result = (self.int_a * self.r_inv) % self.int_n
+        result = self.from_montgomery(self.int_a)
         return [self.format_result(result)]