[gdb/testsuite] Fix some test-cases for check-read1 (gdb_test_lines)

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

Fix some by using gdb_test_lines, as well as related gdb_get_lines.

Tested on x86_64-linux.
7 files changed