tst_QGeo{Path,Polygon}: fix one -Wreturn-type each

GCC complained that "control reaches end of non-void function" in the
construct*() functions. It's correct, since enum values are not
limited to enumerated values.

Add Q_UNREACHABLE_RETURN().

Amends b2edd51351af86381e22e76d139e983dd94d32c5 (picked to 6.8).

Pick-to: 6.12 6.11 6.8
Change-Id: I06e6c86bb431e477ea9a0488b114e5f8b33e358d
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>