Add info for setting environment on IWebHostBuilder (#18313)

Author: Steborino

aspnetcore/test/integration-tests.md

@@ -303,18 +303,25 @@ For more information on `WebApplicationFactoryClientOptions`, see the [Client op
 
 ## Set the environment
 
-By default, the SUT's host and app environment is configured to use the Development environment. To override the SUT's environment:
+By default, the SUT's host and app environment is configured to use the Development environment. To override the SUT's environment when using `IHostBuilder`:
 
 * Set the `ASPNETCORE_ENVIRONMENT` environment variable (for example, `Staging`, `Production`, or other custom value, such as `Testing`).
 * Override `CreateHostBuilder` in the test app to read environment variables prefixed with `ASPNETCORE`.
 
 ```csharp
-protected override IHostBuilder CreateHostBuilder() => 
+protected override IHostBuilder CreateHostBuilder() =>
     base.CreateHostBuilder()
         .ConfigureHostConfiguration(
             config => config.AddEnvironmentVariables("ASPNETCORE"));
 ```
 
+If the SUT uses the Web Host (`IWebHostBuilder`), override `CreateWebHostBuilder`:
+
+```csharp
+protected override IWebHostBuilder CreateWebHostBuilder() =>
+    base.CreateWebHostBuilder().UseEnvironment("Testing");
+```
+
 ## How the test infrastructure infers the app content root path
 
 The `WebApplicationFactory` constructor infers the app [content root](xref:fundamentals/index#content-root) path by searching for a [WebApplicationFactoryContentRootAttribute](/dotnet/api/microsoft.aspnetcore.mvc.testing.webapplicationfactorycontentrootattribute) on the assembly containing the integration tests with a key equal to the `TEntryPoint` assembly `System.Reflection.Assembly.FullName`. In case an attribute with the correct key isn't found, `WebApplicationFactory` falls back to searching for a solution file (*.sln*) and appends the `TEntryPoint` assembly name to the solution directory. The app root directory (the content root path) is used to discover views and content files.