fix(ngMock/$interval): add support for zero-delay intervals in tests

Previously, trying to test code thaat contained zero-delay intervals (e.g.
`$interval(fn, 0)` or `$interval(fn)`) would result in an infinite loop.
This commit avoids the infinite loop, by treating zero-delay intervals as one
second intervals (except for the initial trigger, where they can also be
executed with `$interval.flush(0)`).

Fixes #15952

Closes #15953