[bz-112809] hostname in krb5.conf domain_realm section must not contain square brackets

The `SAML2AttributeMappingTestCase` in `testsuite/integration/basic` used a wrong hostname value in the generated `krb5.conf` file in case of IPv6 address.

This fix strips square brackets from the hostname in the `[domain_realm]` section of the `krb5.conf` file.