Add way to ignore any tmt tests in extracted sources for discover -h fmf --dist-git-source

[lukaszachy]

No description provided.

[LecrisUT]

Sounds rather tricky. What is a situation where this would be needed? How much does it overlap with making dist-git-source a standalone prepare plugin?

Another idea could be to allow to extract dist-git-source into a sub-path, thus any fmf-tree in the sources would not interfere with the parent's tmt-tree.