tst_qicon_svg: add the plugins as dependencies

Before, when you were doing

  ninja qtsvg/tests/all && ctest -L qtsvg/tests/auto/

in a clean builddir, this test would fail in initTestCase() d/t
"no SVG support".

Amends the port to cmake (6.0). I didn't check whether Qt 5 has the
same issue.

Pick-to: 6.12 6.11 6.8
Change-Id: Icdbc8990c99841f6cf2c7df6716ae7876ad4dcac
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>