[wasm][nodejs] Ensure that stdout/stderr have been flushed out before exiting (#70416)

When the results xml is large, and we are writing the base64
representation in one line, `node` can exit before all the output gets
flushed out. This results in xharness getting an incomplete
`STARTRESULTXML <len> <base64> ... ` with missing `ENDRESULTXML`, thus
no `testResults.xml` is generated.

This can be seen in the case of `Microsoft.Extensions.Primitives.Tests`
which has xml ~140KB, and `System.Memory.Tests` which has a xml ~13MB.

So, wait for the two streams to be flushed out, with a timeout of 3secs.

- use the `drain` event only if `stream.write('')` returns `false`
* Address review feedback @kg, @maraf

src/mono/wasm/test-main.js

@@ -105,7 +105,30 @@ function set_exit_code(exit_code, reason) {
         }
 
     } else if (App && App.INTERNAL) {
-        App.INTERNAL.mono_wasm_exit(exit_code);
+        if (is_node) {
+            let _flush = function(_stream) {
+                return new Promise((resolve, reject) => {
+                    if (!_stream.write('')) {
+                        _stream.on('drain', () => resolve());
+                        setTimeout(reject, 3000);
+                    } else {
+                        resolve();
+                    }
+                });
+            };
+            let stderrFlushed = _flush(process.stderr);
+            let stdoutFlushed = _flush(process.stdout);
+
+            Promise.all([ stdoutFlushed, stderrFlushed ])
+                    .then(
+                        () => App.INTERNAL.mono_wasm_exit(exit_code),
+                        reason => {
+                            console.error(`flushing std* streams failed: ${reason}`);
+                            App.INTERNAL.mono_wasm_exit(123);
+                        });
+        } else {
+            App.INTERNAL.mono_wasm_exit(exit_code);
+        }
     }
 }