FindCxxTest: Add support for CxxTest 4 (#13022)

In CxxTest version 4 the Perl generator has been deleted and the
Python generator is installed without the ".py" extension.

Modules/FindCxxTest.cmake

@@ -9,6 +9,7 @@
 #       Only used in the case both Python & Perl
 #       are detected on the system to control
 #       which CxxTest code generator is used.
+#       Valid only for CxxTest version 3.
 #
 #       NOTE: In older versions of this Find Module,
 #       this variable controlled if the Python test
@@ -159,7 +160,8 @@ find_package(PythonInterp QUIET)
 find_package(Perl QUIET)
 
 find_path(CXXTEST_INCLUDE_DIR cxxtest/TestSuite.h)
-find_program(CXXTEST_PYTHON_TESTGEN_EXECUTABLE cxxtestgen.py
+find_program(CXXTEST_PYTHON_TESTGEN_EXECUTABLE
+         NAMES cxxtestgen cxxtestgen.py
          PATHS ${CXXTEST_INCLUDE_DIR})
 find_program(CXXTEST_PERL_TESTGEN_EXECUTABLE cxxtestgen.pl
          PATHS ${CXXTEST_INCLUDE_DIR})
@@ -167,7 +169,7 @@ find_program(CXXTEST_PERL_TESTGEN_EXECUTABLE cxxtestgen.pl
 if(PYTHONINTERP_FOUND OR PERL_FOUND)
    include(${CMAKE_CURRENT_LIST_DIR}/FindPackageHandleStandardArgs.cmake)
 
-   if(PYTHONINTERP_FOUND AND (CXXTEST_USE_PYTHON OR NOT PERL_FOUND))
+   if(PYTHONINTERP_FOUND AND (CXXTEST_USE_PYTHON OR NOT PERL_FOUND OR NOT DEFINED CXXTEST_USE_PYTHON))
       set(CXXTEST_TESTGEN_EXECUTABLE ${CXXTEST_PYTHON_TESTGEN_EXECUTABLE})
       set(CXXTEST_TESTGEN_INTERPRETER ${PYTHON_EXECUTABLE})
       FIND_PACKAGE_HANDLE_STANDARD_ARGS(CxxTest DEFAULT_MSG