System.Net.Ping: Skip TimeoutIsRespected test if network is unreachab…

…le (#58745)

The TestSettings.UnreachableAddress is using 192.0.2.0 which is documented in RFC5735 as a network to be used in docs/example code.
On the Android devices in Helix the upstream network is configured to not route this address which results in a "Destination Net Unreachable" response
which causes the timeout argument to ping to be ignored and ping returns immediately.
Skip the test in these cases.

Example output from ping on the device:

```
1|sunfish:/ $ time ping -c 1 -W 2 -s 50 192.0.2.0
PING 192.0.2.0 (192.0.2.0) 50(78) bytes of data.
From 131.107.5.118: icmp_seq=1 Destination Net Unreachable

--- 192.0.2.0 ping statistics ---
1 packets transmitted, 0 received, +1 errors, 100% packet loss, time Oms

   0m00.03s real   0m00.00s user   0m00.00s system
```

src/libraries/System.Net.Ping/tests/FunctionalTests/UnixPingUtilityTests.cs

@@ -8,6 +8,7 @@
 using System.Threading.Tasks;
 
 using Xunit;
+using Microsoft.DotNet.XUnitExtensions;
 
 namespace System.Net.NetworkInformation.Tests
 {
@@ -19,7 +20,7 @@ public class UnixPingUtilityTests
     {
         private const int IcmpHeaderLengthInBytes = 8;
 
-        [Theory]
+        [ConditionalTheory]
         [InlineData(0)]
         [InlineData(100)]
         [InlineData(1000)]
@@ -32,11 +33,24 @@ public static void TimeoutIsRespected(int timeout)
             p.StartInfo.RedirectStandardError = true;
             p.StartInfo.RedirectStandardOutput = true;
 
+            bool destinationNetUnreachable = false;
+            p.OutputDataReceived += delegate (object sendingProcess, DataReceivedEventArgs outputLine)
+            {
+                if (outputLine.Data?.Contains("Destination Net Unreachable", StringComparison.OrdinalIgnoreCase) == true)
+                    destinationNetUnreachable = true;
+            };
+
             Stopwatch stopWatch = Stopwatch.StartNew();
 
             p.Start();
+            p.BeginOutputReadLine();
             p.WaitForExit();
 
+            if (destinationNetUnreachable)
+            {
+                throw new SkipTestException($"Network doesn't route {TestSettings.UnreachableAddress}, skipping test.");
+            }
+
             //ensure that the process takes longer than or within 10ms of 'timeout', with a 5s maximum
             Assert.InRange(stopWatch.ElapsedMilliseconds, timeout - 10, 5000);
         }