[lldb/test] Reduce API test tools configuration boilerplate

Replace the dotest command line options and various cmake variables,
which are used for passing the locations of llvm tools to the API tests
with a single variable, which points to the directory these tools are
placed in. Besides reducing repetition, this also makes things more
similar to how "normal" llvm tests are configured.

Differential Revision: https://reviews.llvm.org/D95261