ctest: Restore default of no time limit for command-line `-T Test` step

Refactoring in commit 83845184db (cmCTest: Move timing functions from
cmCTestScriptHandler to cmCTest, 2024-10-23, v4.0.0-rc1~576^2) enabled a
time limit even if the undocumented `CTEST_TIME_LIMIT` script variable
is not set.