OutOfMemoryError in test suite when upgrading from JUnit 5.2 to 5.3
## Overview

I work on a large project which runs Junit 5 under Gradle (currently version 4.8).  One of our test suites which previously ran fine with a 8 GB max heap size now requires an ~11 GB heap to run without OutOfMemoryError (~12 GB to run without OutOfMemoryError AND without GC overhead slowing things down significantly).

Looking at the release notes, I see nothing that should cause such a significant increase in memory usage from Junit 5 itself, so I was surprised to see this new problem while upgrading versions.

I only found one other open issue mentioning OutOfMemoryError and it involves dynamic tests, which my suite isn't using at all.

I suppose it's possible that some small behavior change in Junit 5.3 sensitizes an issue with my test suite that increases memory consumption, but I can't think of what that might be. 

Are there any changes in Junit 5.3 that are known to increase memory consumption, and thus increase risk of OutOfMemoryError while running tests?

My apologies if this issue seems more like a question and not a real bug report... but I figured it was better to raise the issue quickly after Junit 5.3 release just in case others are going to start hitting the same problem soon.

Unfortunately, I can't share my unit tests from this project.  However, I am going to try to pare down the test suite to see if I can narrow down where the memory increase is happening.  I may or may not be able to come back here later and provide more specific information, after doing that work.

## Deliverables

**Team Decision**:

- [x] Implement the _quick fix_ mentioned by @sbrannen.
- [x] Investigate other possibilities for freeing up unnecessary memory consumption in the `HierarchicalTestEngine` and related infrastructure (specifically in `NodeTestTask`).
- [x] Backport fix and release notes to a new `5.3.1` branch.