Fix Spring boot deferredResult test flakyness

[jaydeluca]

The deferredResult test (introduced in #15249) is flaky. it fails when run with the entire test suite:

./gradlew :instrumentation:spring:spring-webmvc:spring-webmvc-6.0:javaagent:test

but will pass when run alone:

./gradlew :instrumentation:spring:spring-webmvc:spring-webmvc-6.0:javaagent:test --tests "*deferredResult*"

See 100% flakyness in CI: