compiletest: Less special handling of debugger/test discovery in debuginfo mode

Currently there is special code in compiletest that detects Mode::DebugInfo and responds by creating three different clones of the compiletest config, each configured to use one of cdb/gdb/lldb if the corresponding debugger can be found.

This complicates config handling and test discovery in the top-level parts of compiletest, because parts of the code need to be aware of multiple configs just to handle this one use-case.

In principle, debugger discovery and selection should be handled out in bootstrap, which then invokes compiletest up to three times, with each invocation explicitly specifying a debugger path and debugger kind. This is somewhat analogous to how x test coverage invokes compiletest twice on the same directory with two different modes, though in this case it would instead be the same directory and mode with different debugger settings.

(Given bootstrap's own maintenance headaches, whether this would be a net improvement in the short-term is an open question.)

Start by tracing compiletest's special handling of Mode::DebugInfo and bootstrap's debugger discovery and test invocation paths. Compare this with how x test coverage invokes compiletest multiple times. Done means debugger selection is handled by bootstrap and compiletest no longer needs multiple cloned configs for this case.