Restore CommonUnitTestsPlaceholder (defines the MockCoordTrans target)

MockCoordTrans is a Release 2 GROUP3 target: a header-only static
library defined in Lts/Common/CommonUnitTestsPlaceholder/mocks. As a
header-only lib it has no source file as a ninja edge input, so the
build-closure keep set did not pull its dir in and the prune removed
its CMakeLists (ninja: unknown target MockCoordTrans). Restore the dir
and its add_subdirectory; the smoke tests inside stay behind their
default-OFF options.

qt6-after/qt6-migrated-legatus/Lts/Common/CommonUnitTestsPlaceholder/coordTransTests_smoke.cpp

@@ -0,0 +1,14 @@
+// Stub QtTest smoke test for coordTrans
+#include <QTest>
+
+class CoordTransSmokeTest : public QObject
+{
+    Q_OBJECT
+
+private slots:
+    void stubTest() {
+        QVERIFY(true);
+    }
+};
+
+QTEST_APPLESS_MAIN(CoordTransSmokeTest)