Flaky llvm-mt tests reported in #20902

[Crivella]

Related to

#20902 (comment)
and
#20902 (comment)

I think this is related to the fact that LLVM_ENABLE_LIBXML2 is on by default in the CMake
https://github.com/llvm/llvm-project/blob/21610e3ecc8bc727f99047e544186b35b1291bcd/llvm/CMakeLists.txt#L563

and we are not explicitly disabling it in the easyblock if there is no dependency loaded by EasyBuild in general but only for full_llvm builds
https://github.com/easybuilders/easybuild-easyblocks/blob/f0c6d696a70edb5a5cd81af04843db2acc238511/easybuild/easyblocks/l/llvm.py#L113C5-L113C34

Guess this could cause system dependencies coming into play in the mentioned builds

Will need testing with properly setting LIBXML2 to OFF if not found