Explicitly declare test fixture usage (elastic#48166)

Author: mark-vieira

x-pack/snapshot-tool/build.gradle

@@ -81,6 +81,8 @@ task s3ThirdPartyTests {
 if (useFixture) {
   apply plugin: 'elasticsearch.test.fixtures'
 
+  testFixtures.useFixture()
+
   task writeDockerFile {
     File minioDockerfile = new File("${project.buildDir}/minio-docker/Dockerfile")
     outputs.file(minioDockerfile)