backtrace: Suppress internal errors by libbfd

backtrace: Suppress internal errors by libbfd

At least since Ubuntu 22.04 there are lots of (apparently non-fatal)
"DWARF error: could not find variable specification" errors generated
by bfd_find_nearest_line().  This also happens with addr2line, which
internally uses libbfd.  No idea what the actual issue is (it still
happens with binutils 2.39 in Ubuntu 22.10), but since it doesn't seem
to affect the functionality we need this avoids filling the test logs
with useless messages.