fix flaky test: removing dead states can turn the NFA into a DFA

Instead assert that DFA doesn't turn into an NFA, that's what we care
about.

Closes #14210

lucene/core/src/test/org/apache/lucene/util/automaton/TestAutomaton.java

@@ -880,9 +880,11 @@ public void testRandomFinite() throws Exception {
               newTerms.add(newTerm.toBytesRef());
             }
             terms = newTerms;
-            boolean wasDeterministic1 = a.isDeterministic();
+            boolean wasDeterministic = a.isDeterministic();
             a = Operations.concatenate(Automata.makeString(prefix.utf8ToString()), a);
-            assertEquals(wasDeterministic1, a.isDeterministic());
+            if (wasDeterministic) {
+              assertTrue(a.isDeterministic());
+            }
           }
           break;