REGTEST: mailers: add new test for 'mailers' section

REGTEST: mailers: add new test for 'mailers' section

This test verifies the mailers section works properly by checking that
it sends the proper amount of mails when health-checks are changing and
or marking a server up/down

The test currently fails on all versions of haproxy i tried with varying
results:

- 1.9.0 produces thousands of mails.
- 1.8.14 only sends 1 mail, needs a 200ms 'timeout mail' to succeed
- 1.7.11 only sends 1 mail, needs a 200ms 'timeout mail' to succeed
- 1.6 only sends 1 mail, (does not have the 'timeout mail' setting implemented)