Add pattern argument to django_assert_num_queries fixture

[qarlosh]

This optional argument allows to filter the queries to take into account for the assertion.

It may be useful to check for the count of only certain queries. For example, a use case of mine is to skip SAVEPOINT queries by using:

NO_SAVEPOINT_PATTERN=r'(?!^(ROLLBACK TO |RELEASE )?SAVEPOINT [`"].+[`"]$)'
...
with django_assert_num_queries(n, pattern=NO_SAVEPOINT_PATTERN):
   ...

[bluetech]

It will be better to propose this change to Django's version first.