server-mode: Add option to enable/disable test case explicitly

Add a `CMake_TEST_SERVER_MODE` option that can be set in testing builds
to enable/disable server mode tests explicitly.  This will allow testing
in combination with `CMake_TEST_EXTERNAL_CMAKE` or for server mode to be
built on systems that have a python version that cannot run the test.

CMakeLists.txt

@@ -715,6 +715,9 @@ if(NOT CMake_TEST_EXTERNAL_CMAKE)
 else()
   set(CMake_ENABLE_SERVER_MODE 0)
 endif()
+if(NOT DEFINED CMake_TEST_SERVER_MODE)
+  set(CMake_TEST_SERVER_MODE ${CMake_ENABLE_SERVER_MODE})
+endif()
 
 if(NOT CMake_TEST_EXTERNAL_CMAKE)
   if(NOT CMake_VERSION_IS_RELEASE)

Tests/CMakeLists.txt

@@ -2723,7 +2723,7 @@ ${CMake_BINARY_DIR}/bin/cmake -DDIR=dev -P ${CMake_SOURCE_DIR}/Utilities/Release
   ADD_TEST_MACRO(CMakeCommands.target_compile_definitions target_compile_definitions)
   ADD_TEST_MACRO(CMakeCommands.target_compile_options target_compile_options)
 
-  if(CMake_ENABLE_SERVER_MODE)
+  if(CMake_TEST_SERVER_MODE)
     # The cmake server-mode test requires python for a simple client.
     find_package(PythonInterp QUIET)
     if(PYTHON_EXECUTABLE)