xtask: Move TMT infrastructure to tmt module and refactor YAML generation

Move TMT test runner code from xtask.rs to tmt module:
- `run_tmt()` and `tmt_provision()` functions
- Helper functions for VM management and SSH connectivity
- Related constants

Also refactor `update_integration()` to use serde_yaml::Value for
building YAML structures instead of string concatenation.

Add detailed error reporting for failed TMT tests:
- Assign run IDs using `tmt run --id`
- Display verbose reports with `tmt run -i {id} report -vvv`

Assisted-by: Claude Code (Sonnet 4.5)
Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters