Update Coroutines to 1.6.0, Kotlin API to 1.5 and refactored tests

[heubeck]

• Refactored and cleaned up tests.
• Update Coroutines to 1.6
• Needed to increase language/api version to 1.5 because of method signature changes in Flow that are only supported from 1.5 onwards (causing the test failures of Bump kotlinx-coroutines-core from 1.5.2 to 1.6.0 #788)

[codecov]

Codecov Report

Merging #790 (6decd09) into main (3ab0833) will increase coverage by 0.16%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main     #790      +/-   ##
============================================
+ Coverage     89.92%   90.08%   +0.16%     
- Complexity     3006     3012       +6     
============================================
  Files           377      377              
  Lines         11890    11891       +1     
  Branches       1499     1502       +3     
============================================
+ Hits          10692    10712      +20     
+ Misses          609      596      -13     
+ Partials        589      583       -6     

Impacted Files	Coverage Δ
...main/kotlin/io/smallrye/mutiny/coroutines/Multi.kt	80.95% <ø> (-3.67%)	⬇️
...c/main/kotlin/io/smallrye/mutiny/coroutines/Uni.kt	100.00% <ø> (ø)
...lrye/mutiny/subscription/SerializedSubscriber.java	83.01% <0.00%> (-4.72%)	⬇️
...tiny/operators/multi/MultiBufferWithTimeoutOp.java	73.10% <0.00%> (-1.69%)	⬇️
...java/io/smallrye/mutiny/helpers/Subscriptions.java	79.00% <0.00%> (-1.11%)	⬇️
.../io/smallrye/mutiny/helpers/queues/DrainUtils.java	80.00% <0.00%> (ø)
...a/io/smallrye/mutiny/helpers/BlockingIterable.java	89.79% <0.00%> (+1.02%)	⬆️
...erators/multi/builders/SerializedMultiEmitter.java	82.27% <0.00%> (+1.26%)	⬆️
... and 6 more