[lldb][tests] Take into account all parent's categories when traverse folders upwards

This is needed to not re-write parent's categories by categories of a nested folder,
e.g. commands/expression/completion specify "cmdline" category, however it still belongs
to parent's "expression" category.

The sentinel ".categories" in the test-suite root directory is no longer needed.

Differential Revision: https://reviews.llvm.org/D71905