Ensure that thread is not interrupted.

It is possible that thread might have been interrupted so shutdownAndAwaitTermination might return silently. Earlier I was thinking we would catch any such error on assertions of the output but that's not guaranteed.

PiperOrigin-RevId: 732167368

transpiler/javatests/com/google/j2cl/transpiler/TranspilerTester.java

@@ -572,6 +572,7 @@ public boolean isCancelled() {
     } finally {
       MoreExecutors.shutdownAndAwaitTermination(executorService, 30, SECONDS);
     }
+    assertThat(Thread.currentThread().isInterrupted()).isFalse();
 
     final String[] knownDelayedCalls = {
       "com.google.j2cl.transpiler.frontend.javac.JavacParser.parseFiles",