resolver tests: set 3MB cache size

resolver tests: set 3MB cache size

The default max-cache-size on the system test being enforced to 2MB
(from the -T switch). It means the main cache size is 6/8 of it (1/8
being reserved for ADB and 1/8 for the delegation database).

However, the minimal cache size of the main cache is 2MB, which means
that the main cache size will actually be 2MB anyway when running the system tests.

This was breaking some resolver tests were failing because when the
cache was shared and named reload, the previous cache size was, indeed,
2MB whereas the newly requested size only 6/8 of it (that happened
before the size is bumped to the minimal value).

Set the ns1 of resolver tests max cache size of 3MB, so even 6/8 of the
max cache size is more than the minimum 2MB of the main cache size,
which enable to keep the shared cached between views for this test
between reloads.