Fix ReactorStmtCtx.calculateParentRefcount()

Fix ReactorStmtCtx.calculateParentRefcount()

If we have an empty grouping, which is referenced via augment-uses to a
different module, then if that module is buildEffective()'d first, we
end up with the grouping being swept.

This turns out to be a bug in parentRef determination -- in this
particular case we observe parent being REFCOUNT_NONE and invoke it
recursively -- and since the parent is a module, it itself does not have
a parentRef. This leads to us thinking the grouping does not have a
parentRef, whereas it does.

Update calculateParentRefcount() to recognize parent.refCount == 0 as
parent holding us down.

The problem manifests itself in the test when the models are processed
in order foo.yang, bar.yang -- but not when bar.yang is processed before
foo.yang.

Unfortunately the order of processing is not predictable in
BuildGlobalContext, hence we repeat the test 4 times.

JIRA: YANGTOOLS-1474
Change-Id: Ib3d55fb92d5a6d34230722ab1cd39e8f8831b6fe
Signed-off-by: Sangwook Ha <sangwook.ha@verizon.com>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit ad55ca2b2e72f15d7963cfcb011e4f7610889fce)