Tests: fix date comparison failure on PHP 7.1

Since PHP 7.1, DateTime constructor incorporates microseconds [1].

This causes the testCreateIssueWithRareFields() case to fail comparing
the last_updated field, as the reference DateTime object now contains
microseconds whereas the underlying database fields does not (see failed
Travis build [2]).

1) IssueAddTest::testCreateIssueWithRareFields
Failed asserting that two DateTime objects are equal.
--- Expected
+++ Actual
@@ @@
-2017-01-05T12:28:55.533798+0000
+2017-01-05T12:28:55.000000+0000

The DateTime is now initialized from time() instead, so the microseconds
are set to 0.

Fixes 0022114

[1] http://php.net/manual/en/migration71.incompatible.php
[2] https://travis-ci.org/mantisbt/mantisbt/jobs/189164182