[lldb] Fix TestDataFormatterGenericList

Test is using "next" commands to make progress in the process. D115137
added an additional statement to the program, without adding a command
to step over it. This only seemed to matter for the libc++ flavour of
the test, possibly because libstdc++ list is "empty" in its
uninitialized state.

Since moving with step commands is a treacherous, this patch adds a
run-to-breakpoint command to the test. It only does this for the
affected step, but one may consider doing it elsewhere too.