[browser][wbt] Fix not failing download for `DownloadProgressFinishes…

…` test when randomly picked index is zero (#98985)

src/mono/wasm/testassets/WasmBasicTestApp/App/wwwroot/README.md

@@ -0,0 +1,15 @@
+## WasmBasicTestApp
+
+This is a test application used by various Wasm.Build.Tests. The idea is to share a common behavior (so that we don't have to maintain many test apps) and tweak it for the test case.
+It typically suits scenario where you need more than a plain template app. If the test case is too different, feel free to create another app.
+
+### Usage
+
+The app reads `test` query parameter and uses it to switch between test cases. Entrypoint is `main.js`.
+There is common unit, then switch based on test case for modifying app startup, then app starts and executes next switch based on test case for actually running code.
+
+Some test cases passes additional parameters to differentiate behavior, see `src/mono/wasm/Wasm.Build.Tests/TestAppScenarios`.
+
+### Running out side of WBT
+
+One of the benefits is that you can copy the app out of intree and run the app without running Wasm.Build.Tests with just `dotnet run`.

src/mono/wasm/testassets/WasmBasicTestApp/App/wwwroot/main.js

@@ -33,6 +33,7 @@ switch (testCase) {
                 Math.floor(Math.random() * 5) + 5,
                 Math.floor(Math.random() * 5) + 10
             ];
+            console.log(`Failing test at assembly indexes [${failAtAssemblyNumbers.join(", ")}]`);
             let alreadyFailed = [];
             dotnet.withDiagnosticTracing(true).withResourceLoader((type, name, defaultUri, integrity, behavior) => {
                 if (type === "dotnetjs") {
@@ -45,8 +46,8 @@ switch (testCase) {
                     return defaultUri;
                 }
 
-                assemblyCounter++;
-                if (!failAtAssemblyNumbers.includes(assemblyCounter) || alreadyFailed.includes(defaultUri))
+                const currentCounter = assemblyCounter++;
+                if (!failAtAssemblyNumbers.includes(currentCounter) || alreadyFailed.includes(defaultUri))
                     return defaultUri;
 
                 alreadyFailed.push(defaultUri);