Parallel tests execution

[jponge]

Depends on #769, do not merge yet

Parallel builds (same JVM for the whole suite) work fine on my side but remain flaky in CI / constrained environments. Hence they must be activated with a Maven profile (-Pparallel-tests)

[codecov]

Codecov Report

Merging #770 (c30df63) into main (a62cb00) will increase coverage by 0.11%.
The diff coverage is 100.00%.

❗ Current head c30df63 differs from pull request most recent head 54bd07c. Consider uploading reports for the commit 54bd07c to get more accurate results

@@             Coverage Diff              @@
##               main     #770      +/-   ##
============================================
+ Coverage     90.08%   90.20%   +0.11%     
- Complexity     3010     3023      +13     
============================================
  Files           377      377              
  Lines         11883    11885       +2     
  Branches       1497     1500       +3     
============================================
+ Hits          10705    10721      +16     
+ Misses          602      596       -6     
+ Partials        576      568       -8     

Impacted Files	Coverage Δ
...ava/io/smallrye/mutiny/groups/MultiTimePeriod.java	100.00% <100.00%> (ø)
...utiny/operators/multi/MultiWindowOnDurationOp.java	65.71% <0.00%> (-4.29%)	⬇️
...tiny/operators/multi/MultiBufferWithTimeoutOp.java	74.78% <0.00%> (-0.85%)	⬇️
...smallrye/mutiny/helpers/test/AssertSubscriber.java	89.47% <0.00%> (-0.76%)	⬇️
...mallrye/mutiny/operators/multi/MultiFlatMapOp.java	86.76% <0.00%> (+0.92%)	⬆️
...allrye/mutiny/operators/uni/UniAndCombination.java	89.53% <0.00%> (+1.16%)	⬆️
...erators/multi/builders/BufferItemMultiEmitter.java	92.10% <0.00%> (+1.31%)	⬆️
...subscription/SwitchableSubscriptionSubscriber.java	99.18% <0.00%> (+1.63%)	⬆️
...java/io/smallrye/mutiny/helpers/Subscriptions.java	81.21% <0.00%> (+1.65%)	⬆️
...erators/multi/builders/SerializedMultiEmitter.java	83.54% <0.00%> (+2.53%)	⬆️
... and 2 more