[gdb/testsuite] Require use_gdb_stub == 0 where appropriate

[gdb/testsuite] Require use_gdb_stub == 0 where appropriate

When running with target board native-gdbserver, we run into a number of FAILs
due to use of the start command (and similar), which is not supported when
use_gdb_stub == 1.

Fix this by:
- requiring use_gdb_stub == 0 for the entire test-case, or
- guarding some tests in the test-case with use_gdb_stub == 0.

Tested on x86_64-linux.