Added test of SnapshotTests use of setup methods

Author: egil

src/TestServiceProvider.cs

@@ -44,6 +44,11 @@ public ServiceDescriptor this[int index]
             }
         }
 
+        /// <summary>
+        /// Creates an instance of the <see cref="TestServiceProvider"/> and sets its service collection to the
+        /// provided <paramref name="initialServiceCollection"/>, if any.
+        /// </summary>
+        /// <param name="initialServiceCollection"></param>
         public TestServiceProvider(IServiceCollection? initialServiceCollection = null) : this(initialServiceCollection ?? new ServiceCollection(), false)
         {
         }

tests/Components/TestComponentBaseTest/SnapshotTestTest.razor

@@ -1,13 +1,19 @@
 @inherits TestComponentBase
 @using Shouldly
 
-<SnapshotTest Setup="() => Services">
+@code {
+    class Dep1 : ITestDep { public string Name { get; } = "FOO"; }
+    class Dep2 : IAsyncTestDep { public Task<string> GetData() => Task.FromResult("BAR"); }
+}
+
+<SnapshotTest Setup="() => Services.AddSingleton<ITestDep, Dep1>()"
+              SetupAsync="() => { Services.AddSingleton<IAsyncTestDep, Dep2>(); return Task.CompletedTask; }">
     <TestInput>
         <SimpleWithDeps></SimpleWithDeps>
         <SimpleWithAyncDeps></SimpleWithAyncDeps>
     </TestInput>
     <ExpectedOutput>
-        @inject ITestDep testDep
-<p>@testDep.Name</p>
+        <p>FOO</p>
+        <p>BAR</p>
     </ExpectedOutput>
 </SnapshotTest>