init nestied formatting test case

tests/snippets/builtin_format.py

@@ -27,3 +27,4 @@ assert f'{100:_}' == '100'
 assert f'{1024:_}' == '1_024'
 assert f'{65536:,}' == '65,536'
 assert f'{4294967296:,}' == '4,294,967,296'
+assert 'F' == "{0:{base}}".format(15, base="X")