[lldb][testsuite] Create a SBDebugger instance for each test

Some tests set settings and don't clean them up, this leads to side effects in other tests.
The patch removes a global debugger instance with a per-test debugger to avoid such effects.

From what I see, lldb.DBG was needed to determine the platform before a test is run,
lldb.selected_platform is used for this purpose now. Though, this required adding a new function
to the SBPlatform interface.

Differential Revision: https://reviews.llvm.org/D74903