Fix issues

[JanCaha]

Fixes several slight related issues:

• main issue was in merge which did not run if the output was copc.laz that also caused issue in other tools as some of them use Merge as last step
• for VPC there as an issue if the files in VPC were .copc.laz where the extensions were not stripped correctly for this the fs::path fileStem(const std::string &filename) was introduced which should only return stem of file without any extensions - this avoids potentialy creating files with .copc.las extension that cannot be read properly by PDAL
• tests that looked really similary were parametrized and some test cases were added (mostly with input with VPC containing .copc.laz files and output of algs to .copc.laz)

Fixes qgis/QGIS#62415 and possibly similar issues