Capture (peak?) memory usage of test runs

[DanTup]

It was request at https://github.com/Dart-Code/Dart-Code/issues/5422 to show the memory usage of tests similar to how we show their durations. However, this information is not captured or available to Dart-Code to show and would need to be captured by pkg:test.

In order to show this, we'd also need support from VS Code so https://github.com/microsoft/vscode/issues/243468 is a pre-requisite (for VS Code, at least).

(cc @stephane-archer - feel free to add any details I may have missed)