Reduce tests flakiness, especially in CI / constrained environments

[jponge]

Work to reduce flakiness of the tests suite.

[codecov]

Codecov Report

Merging #769 (007b15d) into main (4bc9baf) will increase coverage by 0.44%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main     #769      +/-   ##
============================================
+ Coverage     89.81%   90.26%   +0.44%     
- Complexity     2999     3018      +19     
============================================
  Files           377      377              
  Lines         11880    11883       +3     
  Branches       1496     1499       +3     
============================================
+ Hits          10670    10726      +56     
+ Misses          614      587      -27     
+ Partials        596      570      -26     

Impacted Files	Coverage Δ
...smallrye/mutiny/helpers/test/AssertSubscriber.java	89.47% <0.00%> (-0.76%)	⬇️
...c/main/kotlin/io/smallrye/mutiny/coroutines/Uni.kt	100.00% <0.00%> (ø)
...iny/operators/multi/builders/BaseMultiEmitter.java	83.33% <0.00%> (ø)
...mallrye/mutiny/operators/multi/MultiFlatMapOp.java	86.15% <0.00%> (+0.61%)	⬆️
...subscription/SwitchableSubscriptionSubscriber.java	97.54% <0.00%> (+0.81%)	⬆️
...java/io/smallrye/mutiny/helpers/Subscriptions.java	81.76% <0.00%> (+1.10%)	⬆️
...allrye/mutiny/operators/uni/UniAndCombination.java	89.53% <0.00%> (+1.16%)	⬆️
...tiny/operators/multi/MultiBufferWithTimeoutOp.java	74.78% <0.00%> (+1.68%)	⬆️
...io/smallrye/mutiny/operators/uni/UniMemoizeOp.java	95.04% <0.00%> (+1.98%)	⬆️
...erators/multi/builders/SerializedMultiEmitter.java	83.54% <0.00%> (+2.53%)	⬆️
... and 5 more

[jponge]

I've tested multiple times on macOS, Github actions and Linux on WSL. Seems ok, I encourage reviewers to check locally as well.

[jponge]

Merging this one, it's been quite reliable in many recent CI runs.