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_gtest.cpp

@@ -0,0 +1,11 @@
+// Stub GTest smoke test for coordTrans
+#include <gtest/gtest.h>
+
+TEST(CoordTransSmokeTest, StubTest) {
+    EXPECT_TRUE(true);
+}
+
+int main(int argc, char **argv) {
+    ::testing::InitGoogleTest(&argc, argv);
+    return RUN_ALL_TESTS();
+}