[gdb/testsuite] Fix some gdb.dwarf2 test-cases for check-read1

I ran the testsuite in an environment simulating a stressed system in
combination with check-read1.  This exposes a few more FAILs.

Fix the gdb.dwarf2 ones by using pipe / grep to filter out unnecessary output.

Tested on x86_64-linux.
5 files changed