salt.modules.parted.list_ tweaks and tests

* Removed listdir based tests per #15950
* Changed call to `lsblk` to not use absolute path
* Reduced code duplication in tests via setUp method
* Refactored and cleaned up list_ function:
  * renamed `comps` variable to `cols` for readability
  * removed try/except from being used as flow control
  * raise exception on parsing errors
  * properly handle headers for not `BYT` parted output