Tweak minidebuginfo-set-and-hit-breakpoint.test

On my system, llvm-objcopy was refusing to remove the .dynsym section
because it was still referenced from .rela.plt. Remove that section too,
and clarify that this is needed only because llvm-objcopy
--only-keep-debug does not work (does not set the sections to
SHT_NOBITS). Also, ensure that the test is not creating temporary files
in the source tree.

llvm-svn: 374046