Increase TTL in SendPingWithLowTtl_RoundtripTimeIsNonZero to fix macOS flakiness

[rzikm]

SendPingWithLowTtl_RoundtripTimeIsNonZero has been consistently failing on macOS since it was introduced (Mar 11, 29 failures across 17 builds on both osx.15.amd64 and osx.26.arm64).

Root Cause

The test sends a ping with TTL=1 and asserts that the TtlExpired reply has RoundtripTime > 0. On macOS, the raw socket Ping implementation computes RTT via:

long roundTripTime = (long)Stopwatch.GetElapsedTime(startingTimestamp).TotalMilliseconds;

The (long) cast truncates sub-millisecond values to 0. On CI machines, the first-hop router responds to TTL=1 pings in under 1ms, so all 3 retry attempts report RoundtripTime == 0.

This doesn't affect Windows, which uses IcmpSendEcho with OS-reported RTT values.

Fix

Increase TTL from 1 to 5 so the ICMP packet travels further before expiring, increasing the likelihood of a measurable (>0ms) round-trip time.

Example failure log: https://helixr1107v0xdeko0k025g8.blob.core.windows.net/dotnet-runtime-refs-heads-main-3b34e246c46a48c489/System.Net.Ping.Functional.Tests/3/console.52343950.log?helixlogtype=result

[rzikm]

/azp run runtime-libraries coreclr-outerloop

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[rzikm]

/azp runtime-coreclr libraries-outerloop

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[rzikm]

/azp run runtime-coreclr libraries-outerloop

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/azp list

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[rzikm]

/azp run runtime-libraries-coreclr outerloop

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[Copilot]

Pull request overview

Adjusts a flaky System.Net.Ping OuterLoop test on macOS by increasing the TTL used for a TtlExpired/TimeExceeded scenario so the measured round-trip time is more likely to be non-zero.

Changes:

• Increase TTL in SendPingWithLowTtl_RoundtripTimeIsNonZero from 1 to 5.
• Add explanatory comments describing the macOS raw-socket RTT truncation behavior and why a higher TTL helps.

Comments suppressed due to low confidence (1)

src/libraries/System.Net.Ping/tests/FunctionalTests/PingTest.cs:832

• Setting options.Ttl = 5 can cause the ping to reach the configured external host when it is within <=5 hops, which would produce IPStatus.Success and fail the assertion that requires TimeExceeded/TtlExpired. To keep this test robust across network topologies, consider dynamically selecting a TTL that results in a non-Success status (e.g., probe with increasing TTL to find hop count and then use a smaller TTL), or skip the test when the host is too close for the chosen TTL.

            // Use TTL=5 instead of 1 to increase the chance of a measurable (>0ms) round-trip
            // time. On macOS, the raw socket implementation truncates sub-millisecond RTT to 0,
            // and the first-hop router often responds in <1ms.
            options.Ttl = 5;
            bool gotNonZeroRtt = false;
            for (int attempt = 0; attempt < 3; attempt++)
            {
                PingReply pingReply = await ping.SendPingAsync(host, TestSettings.PingTimeout, payload, options);

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