ctest: print error for invalid CTEST_CUSTOM_TEST_OUTPUT_TRUNCATION

Print an error message for invalid values of
`CTEST_CUSTOM_TEST_OUTPUT_TRUNCATION` in a ctest dashboard script.
The option was added by commit 140704d443 (ctest: add option for
output truncation, 2022-03-07, v3.24.0-rc1~513^2).

Fixes: #23869