test: pin the offscreen platform, and let the popup test check its own geometry

ctest sets no QT_QPA_PLATFORM, so the suite's verdict depended on how it
was invoked: green for anyone exporting `offscreen` by hand, red under
ctest in the same tree. That cost a wrong diagnosis before the cause was
found, an unrelated change blamed for a failure that reproduces on a
clean checkout.

The Wayland warning blames a missing transientParent, which is
misleading. Instrumenting the test showed the popup viewport arriving as
1278x0. The zero height is why the grab returned a null pixmap, but the
width is the worse half: this test sizes a line edit to 550px and exists
to prove a description survives a popup that size, so a working grab
would have measured a popup twice that wide and passed while proving
nothing. Offscreen gives 548x40, the geometry the test means.

Pinning the platform is therefore the correct fix rather than an opt-out.
It is set in add_qtmaildir_test so it covers every test, including the
two others that create widgets, and anything added later.

The test now also asserts its own geometry, since CMake only governs
ctest and the binary is often run directly. A bare !shot.isNull() said
nothing useful; it now reports "popup viewport has no height (1278x0)"
or names a popup too wide to be the case under test. Both guards were
verified by mutation.

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