Tests: Run FindGTK2 tests only when explicitly enabled

Do not `find_package(GTK2)` by default, as it gives the impression that
CMake depends on it to build.  We will set `CMake_TEST_FindGTK2` in the
cache of automated builds where GTK2 is expected to work.

Tests/CMakeLists.txt

@@ -1454,6 +1454,10 @@ ${CMake_BINARY_DIR}/bin/cmake -DDIR=dev -P ${CMake_SOURCE_DIR}/Utilities/Release
     add_subdirectory(GoogleTest)
   endif()
 
+  if(CMake_TEST_FindGTK2)
+    add_subdirectory(FindGTK2)
+  endif()
+
   if(CMake_TEST_FindIconv)
     add_subdirectory(FindIconv)
   endif()
@@ -1597,11 +1601,6 @@ ${CMake_BINARY_DIR}/bin/cmake -DDIR=dev -P ${CMake_SOURCE_DIR}/Utilities/Release
     ADD_TEST_MACRO(FindMatlab.r2018a_check   ${CMAKE_CTEST_COMMAND} -V -C $<CONFIGURATION>)
   endif()
 
-  find_package(GTK2 QUIET)
-  if(GTK2_FOUND)
-    add_subdirectory(FindGTK2)
-  endif()
-
   add_test(ExternalProject ${CMAKE_CTEST_COMMAND}
     --build-and-test
     "${CMake_SOURCE_DIR}/Tests/ExternalProject"

Tests/RunCMake/CMakeLists.txt

@@ -339,8 +339,7 @@ if(PKG_CONFIG_FOUND)
   add_RunCMake_test(FindPkgConfig)
 endif()
 
-find_package(GTK2 QUIET)
-if (GTK2_FOUND)
+if(CMake_TEST_FindGTK2)
   add_RunCMake_test(FindGTK2)
 endif()