Use simpler int to hex string conversion

Signed-off-by: Werner Lewis <werner.lewis@arm.com>
diff --git a/tests/scripts/generate_bignum_tests.py b/tests/scripts/generate_bignum_tests.py
index 2a81077..cc4db4c 100755
--- a/tests/scripts/generate_bignum_tests.py
+++ b/tests/scripts/generate_bignum_tests.py
@@ -228,7 +228,7 @@
         self.symbol = "+"
 
     def result(self) -> str:
-        return quote_str(hex(self.int_l + self.int_r).replace("0x", "", 1))
+        return quote_str("{:x}".format(self.int_l + self.int_r))
 
 
 if __name__ == '__main__':