pmg-smtp-filter: rename proxtest.com to pmg.example

pmg-smtp-filter: rename proxtest.com to pmg.example

the proxtest.com addresses have been hardcoded for a long while, and
are used for the regression tests.

While the regression tests do not send out mails - certain
configurations might - e.g.:
```
section: mail
        ndr_on_block 1
```

noticed while running the regression tests with this setting and
seeing that my queue had messages, which could not be delivered due to
a time-out

replace the addresses with `pmg.example` (longer names make the
test-output wrap to multiple lines causing more churn in the
regressions tests) - pmg.example is safe as .example is reserved (for
such purposes) [0]

[0] https://en.wikipedia.org/wiki/.example

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>