Fix a division-by-zero crash in the VPN code

[diegoreymendez]

Task/Issue URL: https://app.asana.com/0/1203137811378537/1207299387361586/f

iOS PR: duckduckgo/iOS#2862
macOS PR: duckduckgo/macos-browser#2785
What kind of version bump will this require?: Patch

Description

Fixes a crash caused by a division by zero.

Testing

See platform-specific PRs for testing instructions.

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Internal references:

Software Engineering Expectations
Technical Design Template

[diegoreymendez]

Don't want to divide by zero.

[mallexxx]

Probably that should work, unless you do a horse kick test 😀

        let d = Date()
        let (rx, tx) = NetworkProtectionConnectionBandwidthAnalyzer.bytesPerSecond(newer: .init(rxBytes: 9_999_999_999_999, txBytes: 9_999_999_999_999, date: d.addingTimeInterval(0.0000001)),
                                                                                   older: .init(rxBytes: 0, txBytes: 0, date: d))

        let rxThreshold = 100 * 1024 // 100k
        let txThreshold = 100 * 1024 // 100k

        let uint_rx = UInt64(rx) // this will overflow
        let uint_tx = UInt64(tx)

But probably this condition is not something that has really happened, because lesser numbers work. Probably the crash was really about the division by zero leading to a NaN to UInt64 conversion.
Anyways, consider adding couple of small sanity unit tests

[diegoreymendez]

I made the method static and added some tests to validate all is good and prevent regressions in the future. I also changed the line that was suffering in readability, I agree. Thanks for the suggestions.

The only suggestion I'm skipping is the larger inline code suggestion in your comment because IMO it favors a lower line count in detriment of readability and maintainability.

I want to make sure the reader can skim through the code and understand quickly, and I think this line isn't good to that end:

let (rx, tx) = NetworkProtectionConnectionBandwidthAnalyzer.bytesPerSecond(newer: .init(rxBytes: 9_999_999_999_999, txBytes: 9_999_999_999_999, date: d.addingTimeInterval(0.0000001)),
                                                                                   older: .init(rxBytes: 0, txBytes: 0, date: d))