tests: speed up testInsertAfterExpand

The test created a file in a collapsed subtree and then waited five
seconds for an items-inserted signal that must never arrive, spending
that whole time on every run and making kfileitemmodeltest about three
times slower than it needs to be.

Create a file in the watched top directory instead and wait for its
insertion. Once that arrives the directory change has been processed and
the model has settled, so the collapsed file has had its chance to appear
and only the expected items are present. The sentinel stays as a
top-level sibling, so the later re-expansion expects it alongside the
restored subtree.

testInsertAfterExpand drops from about 4.9 seconds to 1.6 seconds and the
whole kfileitemmodeltest from about 7.2 seconds to 2.4 seconds.

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