Enable test-fixture test suites

Today the `:test:fixtures` modules' test suites are disabled, but in
fact these fixtures do have nontrivial behaviour that wants testing in
its own right, so we should run their tests.

This commit reinstates the disabled tests and fixes one which should
have been fixed as part of elastic#116212.

test/fixtures/build.gradle

@@ -1,9 +0,0 @@
-
-subprojects {
-  // fixtures don't have tests, these are external projects used by the build
-  pluginManager.withPlugin('java') {
-    tasks.named('test').configure {
-      enabled = false
-    }
-  }
-}

test/fixtures/s3-fixture/src/test/java/fixture/s3/S3HttpHandlerTests.java

@@ -31,6 +31,8 @@
 import java.util.List;
 import java.util.Objects;
 
+import static org.hamcrest.Matchers.allOf;
+import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.greaterThan;
 
 public class S3HttpHandlerTests extends ESTestCase {
@@ -261,7 +263,7 @@ public void testListAndAbortMultipartUpload() {
             <Delimiter /><Prefix>path/blob</Prefix><MaxUploads>10000</MaxUploads><IsTruncated>false</IsTruncated>\
             </ListMultipartUploadsResult>"""), handleRequest(handler, "GET", "/bucket/?uploads&prefix=path/blob"));
 
-        assertEquals(RestStatus.NOT_FOUND, handleRequest(handler, "POST", "/bucket/path/blob?uploadId=" + uploadId, Strings.format("""
+        final var completeUploadResponse = handleRequest(handler, "POST", "/bucket/path/blob?uploadId=" + uploadId, Strings.format("""
             <?xml version="1.0" encoding="UTF-8"?>
             <CompleteMultipartUpload xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
                <Part>
@@ -272,7 +274,13 @@ public void testListAndAbortMultipartUpload() {
                   <ETag>%s</ETag>
                   <PartNumber>2</PartNumber>
                </Part>
-            </CompleteMultipartUpload>""", part1Etag, part2Etag)).status());
+            </CompleteMultipartUpload>""", part1Etag, part2Etag));
+        if (completeUploadResponse.status() == RestStatus.OK) {
+            // possible, but rare, indicating that S3 started processing the upload before returning an error
+            assertThat(completeUploadResponse.body().utf8ToString(), allOf(containsString("<Error>"), containsString("NoSuchUpload")));
+        } else {
+            assertEquals(RestStatus.NOT_FOUND, completeUploadResponse.status());
+        }
     }
 
     private static String getUploadId(BytesReference createUploadResponseBody) {