[Test] Fix SharedBlobCacheServiceTests.testGetMultiThreaded (#112322)

A cacheFileRegion can be concurrently evicted while its being incref'd.

See this comment
https://github.com/elastic/elasticsearch/blob/98fe686da4c5cb82d4b03719977be428dc7934e7/x-pack/plugin/blob-cache/src/main/java/org/elasticsearch/blobcache/shared/SharedBlobCacheService.java#L1812-L1813

The tryRead method also performs null and eviction check for io before
returing true.
https://github.com/elastic/elasticsearch/blob/98fe686da4c5cb82d4b03719977be428dc7934e7/x-pack/plugin/blob-cache/src/main/java/org/elasticsearch/blobcache/shared/SharedBlobCacheService.java#L926-L931

Resolves: #112314

Author: ywangd

muted-tests.yml

@@ -154,9 +154,6 @@ tests:
 - class: org.elasticsearch.search.retriever.rankdoc.RankDocsSortBuilderTests
   method: testEqualsAndHashcode
   issue: https://github.com/elastic/elasticsearch/issues/112312
-- class: org.elasticsearch.blobcache.shared.SharedBlobCacheServiceTests
-  method: testGetMultiThreaded
-  issue: https://github.com/elastic/elasticsearch/issues/112314
 - class: org.elasticsearch.search.retriever.RankDocRetrieverBuilderIT
   method: testRankDocsRetrieverWithCollapse
   issue: https://github.com/elastic/elasticsearch/issues/112254

x-pack/plugin/blob-cache/src/test/java/org/elasticsearch/blobcache/shared/SharedBlobCacheServiceTests.java

@@ -444,7 +444,6 @@ public void testMassiveDecay() throws IOException {
      * Exercise SharedBlobCacheService#get in multiple threads to trigger any assertion errors.
      * @throws IOException
      */
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/112305")
     public void testGetMultiThreaded() throws IOException {
         final int threads = between(2, 10);
         final int regionCount = between(1, 20);
@@ -494,11 +493,11 @@ public void testGetMultiThreaded() throws IOException {
                                     assert allowAlreadyClosed || e.getMessage().equals("evicted during free region allocation") : e;
                                     throw e;
                                 }
+                                assertTrue(cacheFileRegion.testOnlyNonVolatileIO() != null || cacheFileRegion.isEvicted());
                                 if (incRef && cacheFileRegion.tryIncRef()) {
                                     if (yield[i] == 0) {
                                         Thread.yield();
                                     }
-                                    assertNotNull(cacheFileRegion.testOnlyNonVolatileIO());
                                     cacheFileRegion.decRef();
                                 }
                                 if (evict[i] == 0) {