tests: Preserve empty arguments in test command lines

This will now preserve empty values in the TEST_LAUNCHER and
CROSSCOMPILING_EMULATOR target properties for tests added by:

- The add_test() command.
- The ExternalData_Add_Test() command from the ExternalData module.
- The gtest_add_tests() or gtest_discover_tests() commands from the
  GoogleTest module.

For the gtest_add_tests() and gtest_discover_tests() commands,
empty elements in the values passed after the EXTRA_ARGS keyword
are also now preserved.

Policy CMP0178 is added to provide backward compatibility with the
old behavior where empty values were silently discarded from the
above cases.

Fixes: #26337

Modules/FindMatlab.cmake

@@ -1012,6 +1012,10 @@ function(matlab_add_unit_test)
     endif()
   endif()
 
+  # The ${${prefix}_TEST_ARGS} and ${${prefix}_UNPARSED_ARGUMENTS} used below
+  # should have semicolons escaped, so empty arguments should be preserved.
+  # There's also no target used for the command, so we don't need to do
+  # anything here for CMP0178.
   add_test(NAME ${${prefix}_NAME}
            COMMAND ${CMAKE_COMMAND}
             "-Dtest_name=${${prefix}_NAME}"