Fix TestFormatters.py stepping too far

TestFormatters.py has a sequence of three 'next' commands to get past
all the initializations in the test function. On AArch64 (and
potentially other platforms), this was one 'next' too many and we ended
up outside our frame.

This patch replaces the sequence with a 'thread until ' the line of the
return from the function, so we should stop after all the
initializations but before actually returning.

Differential Revision: https://reviews.llvm.org/D70303