lldb tests are only run on macOS

[tromey]

In #54004 I discovered that lldb tests are only run on macOS:

                return if builder.config.build.contains("apple") {
...

I think it would be better to test lldb and gdb based on availability, not build OS.

[tromey]

I have a patch but it depends on #54764, so I'm waiting for that to land first.