Prior to this commit, on calling `RESET`/`RESET ALL` command, the value of guc
was not getting updated on all the slices(already spun up reader slices).
This commit fixes the issue.
Signed-off-by: NEkta Khanna <ekhanna@pivotal.io>

In order to test this, modify the gpdiff tool to not suppress duplicate
WARNINGs, if the duplicates don't have a segment number attached to them.
The bug was easy to see by setting work_mem, which emits a WARNING in
GPDB, except that suppressing the duplicates hid the bug. Even though we
have a few tests that set work_mem, add one that does that explicitly for
the purpose of testing for this bug, with a comment explaining its purpose.

While we're at it, move the GPDB-specific test queries out of 'guc' test,
into a new 'guc_gp' test, to keep the upstream 'guc' test pristine.

Fixes github issue #3008, reported by @guofengrichard.