[lldb][tests] Make it possible to expect failure for a whole category

There already are decorators and "--excluded" option to mark test-cases/files
as expected to fail. However, when a new test file is added and it which relates
to a feature that a target doesn't support, this requires either adding decorators
to that file or modifying the file provided as "--excluded" option value.

The purpose of this patch is to avoid any modifications in such cases.
E.g. if a target doesn't support "watchpoints" and passes "--xfail-category watchpoint"
to dotest, a testing job will not fail after a new watchpoint-related test file is added.

Differential Revision: https://reviews.llvm.org/D71906