Help/ctest: Document CTEST_SCRIPT_NAME

This variable has existed for a long time without documentation. While
we're here, add details to the CTEST_SCRIPT_DIRECTORY doc.

Help/manual/cmake-variables.7.rst

@@ -818,6 +818,7 @@ Variables for CTest
    /variable/CTEST_RESOURCE_SPEC_FILE
    /variable/CTEST_RUN_CURRENT_SCRIPT
    /variable/CTEST_SCRIPT_DIRECTORY
+   /variable/CTEST_SCRIPT_NAME
    /variable/CTEST_SITE
    /variable/CTEST_SOURCE_DIRECTORY
    /variable/CTEST_SUBMIT_INACTIVITY_TIMEOUT

Help/variable/CTEST_SCRIPT_DIRECTORY.rst

@@ -1,5 +1,9 @@
 CTEST_SCRIPT_DIRECTORY
 ----------------------
 
-The directory containing the top-level CTest script.
-The concept is similar to :variable:`CMAKE_SOURCE_DIR`.
+The directory component of the path to the top-level CTest script specified by
+the :option:`ctest -S` option.  It is given as it appears in that path, so it
+may be relative.
+
+The file name part is available in :variable:`CTEST_SCRIPT_NAME`.  The concept
+is similar to :variable:`CMAKE_CURRENT_LIST_DIR`.

Help/variable/CTEST_SCRIPT_NAME.rst

@@ -0,0 +1,6 @@
+CTEST_SCRIPT_NAME
+-----------------
+
+The file name, without any directory component, of the top-level CTest
+script specified by the :option:`ctest -S` option.  The directory part is
+available in :variable:`CTEST_SCRIPT_DIRECTORY`.