ctest: Restore support for -T Test without dashboard configuration

Since commit 774fcbe4 (CTest: Base command line mode on top of
scripting commands, 2024-10-05, v4.0.0-rc1~653^2) we drive dashboard
client steps through `cmCTestHandlerCommand::ExecuteHandlerCommand`
instead of calling `ProcessHandler` directly.  This requires the
`BuildDirectory` to be known to establish a work directory.

Fixes: #26743
Co-authored-by: Brad King <brad.king@kitware.com>