[wasm-browser][tests] Fix issue with some tests exiting with no output (

#300)

The problem here is that these tests run very quickly, writing to the
console.log very fast, and selenium seems to not return the logs during
that time. So, when the tests are done, which is indicated by an html
element being set, we stop listening for more log messages.
And exit with the test exit code, but no output messages from the test
run.

Eg: `System.Data.Common`

src/Microsoft.DotNet.XHarness.CLI/Commands/WASM/Browser/WasmBrowserTestRunner.cs

@@ -37,6 +37,8 @@ internal class WasmBrowserTestRunner
         // Eg. `foo` becomes `http://localhost:8000/xyz.js 0:12 "foo"
         static readonly Regex s_consoleLogRegex = new Regex(@"^\s*[a-z]*://[^\s]+\s+\d+:\d+\s+""(.*)""\s*$", RegexOptions.Compiled);
 
+        private bool _endResultXmlStringSeen = false;
+
         public WasmBrowserTestRunner(WasmTestBrowserCommandArguments arguments, IEnumerable<string> passThroughArguments,
                                             Action<string> processLogMessage, ILogger logger)
         {
@@ -81,12 +83,19 @@ public async Task<ExitCode> RunTestsWithWebDriver(IWebDriver driver)
                     throw logPumpingTask.Exception!;
                 }
 
-                // WebDriverWait could return before we could get all
-                // the messages, so pump any remaining ones
-                PumpLogMessages(driver);
-
                 if (task == testsDoneTask && testsDoneTask.IsCompletedSuccessfully)
                 {
+                    // WebDriverWait could return before we could get all
+                    // the messages, so pump any remaining ones.
+                    // This can also be a bit delayed, if the console is very
+                    // frequently being written to, in the browser
+                    int count = 0;
+                    do
+                    {
+                        await Task.Delay(500);
+                        PumpLogMessages(driver);
+                    } while (count < 5 && !_endResultXmlStringSeen);
+
                     var elem = testsDoneTask.Result;
                     if (int.TryParse(elem.Text, out var code))
                     {
@@ -96,6 +105,17 @@ public async Task<ExitCode> RunTestsWithWebDriver(IWebDriver driver)
                     return ExitCode.RETURN_CODE_NOT_SET;
                 }
 
+                if (task.IsFaulted)
+                {
+                    _logger.LogDebug(task.Exception!, "Waiting for tests failed");
+                    throw task.Exception!;
+                }
+
+                // WebDriverWait could return before we could get all
+                // the messages, so pump any remaining ones.
+                await Task.Delay(1000);
+                PumpLogMessages(driver);
+
                 return ExitCode.TIMED_OUT;
             }
             finally
@@ -148,6 +168,14 @@ private void PumpLogMessages(IWebDriver driver)
                 msg += Environment.NewLine;
 
                 _processLogMessage(msg);
+
+                // the test runner writes this as the last line,
+                // after the tests have run, and the xml results file
+                // has been written to the console
+                if (msg.Contains("ENDRESULTXML"))
+                {
+                    _endResultXmlStringSeen = true;
+                }
             }
         }