dnsdist: Fix regression tests with Python 3.13

dnsdist: Fix regression tests with Python 3.13

The CA certificates that we are generating as par of our regression tests
were lacking the X.509 `Key Usage` extension, causing TLS validation with
Python 3.13 to fail with:

> certificate verify failed: CA cert does not include key usage extension

It appears that Python 3.13 enables `VERIFY_X509_STRICT` by default, which makes OpenSSL stricter, and thus it chokes on our invalid CA.

(cherry picked from commit 98ef889a9232877ac369b67c11922bab92ab84a9)
(cherry picked from commit 5787ffad38d920d972c683e5ef500a595aca4fec)