Optimize Dovecot configuration for IMAP tests

[Girgias]

This halves the time taken by the SKIPIF section with parallel testing when the ext/imap/tests/imap_errors_basic.phpt is running (and possibly others) as it's hogging the connections by providing an invalid password.

Another possibility is to reduce the number of attempts in the SKIPIF section, as it turns out the retries parameter, actually corresponds to "attempts" and using 0, just retries until the mail-server terminates the connection after too many retries/too long.

@cmb69 could you test that the modification to the test still work on Windows?

[cmb69]

Looks good here:

Number of tests :   82                81
Tests skipped   :    1 (  1.2%) --------
Tests warned    :    0 (  0.0%) (  0.0%)
Tests failed    :    0 (  0.0%) (  0.0%)
Tests passed    :   81 ( 98.8%) (100.0%)