Add semaphore to selenium tests

This change prevents thread pool starvation when running a bunch of
selenium-based tests, by turning the blocking wait for a WebDriver to
start into an async wait.

This also seems to help with speed, and reliability since we're not
running too many browsers at once. I was experencing timeouts, and
seeing them in the debugger while running tests locally, this no longer
happens.

src/Shared/E2ETesting/BrowserTestBase.cs

@@ -21,6 +21,11 @@ namespace Microsoft.AspNetCore.E2ETesting
         private static readonly AsyncLocal<ILogs> _logs = new AsyncLocal<ILogs>();
         private static readonly AsyncLocal<ITestOutputHelper> _output = new AsyncLocal<ITestOutputHelper>();
 
+        // Limit the number of concurrent browser tests.
+        private readonly static int MaxConcurrentBrowsers = Environment.ProcessorCount * 2;
+        private static readonly SemaphoreSlim _semaphore = new SemaphoreSlim(MaxConcurrentBrowsers);
+        private bool _semaphoreHeld;
+
         public BrowserTestBase(BrowserFixture browserFixture, ITestOutputHelper output)
         {
             BrowserFixture = browserFixture;
@@ -39,6 +44,11 @@ namespace Microsoft.AspNetCore.E2ETesting
 
         public Task DisposeAsync()
         {
+            if (_semaphoreHeld)
+            {
+                _semaphore.Release();
+            }
+
             return Task.CompletedTask;
         }
 
@@ -49,6 +59,9 @@ namespace Microsoft.AspNetCore.E2ETesting
 
         public virtual async Task InitializeAsync(string isolationContext)
         {
+            await _semaphore.WaitAsync(TimeSpan.FromMinutes(30));
+            _semaphoreHeld = true;
+
             var (browser, logs) = await BrowserFixture.GetOrCreateBrowserAsync(Output, isolationContext);
             _asyncBrowser.Value = browser;
             _logs.Value = logs;