tests: gdbstub: Fix incomplete code coverage dump

When the test runs with code coverage, make sure the coverage dump
completes its output under pytest execution before the test debug
session ends in both cases: for Zephyr gdbstub feature and for
Qemu internal GDB stub.

Add few more test operations with breakpoints.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>