selftests: netconsole: Do not exit from inside the validation function

Remove the exit call from validate_result() function and move the
test exit logic to the main script. This allows the function to
be reused in scenarios where the test needs to continue execution
after validation, rather than terminating immediately.

The validate_result() function should focus on validation logic
only, while the calling script maintains control over program
flow and exit conditions. This change improves code modularity
and prepares for potential future enhancements where multiple
validations might be needed in a single test run.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250609-netcons_ext-v3-3-5336fa670326@debian.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>