test: Add timeouts and enable nrf9160dk HIL testing

[szczys]

• Add optional command line arguments to verify.py for increasing the serial connection timeout
• Add optional Kconfig symbol to samples/test/src/main.c to increase timeout
• Enable nrf9160 workflow but only on schedule and manual so connectivity doesn't block/slow down merging

[github-actions[bot]]

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[szczys]

@mniestroj thanks for the help with the argparse default settings. I've incorporated your suggestions and updated the commit messages as requested.