Use JUnit's assert format for assert messages to enable better suppor…

[bjoernQ]

This changes the message format of "assert" to match that of JUnit. This way IDEs like IntelliJ IDEA are able to parse it and offer functionality like "compare values".

[akarnokd]

I thought IntelliJ already recognized RxJava's Expected/Actual after this: #5258.

[vanniktech]

I thought so too.

[codecov]

Codecov Report

Merging #6262 into 2.x will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6262      +/-   ##
============================================
+ Coverage     98.23%   98.27%   +0.03%     
  Complexity     6203     6203              
============================================
  Files           667      667              
  Lines         44889    44889              
  Branches       6216     6216              
============================================
+ Hits          44097    44113      +16     
+ Misses          251      247       -4     
+ Partials        541      529      -12

Impacted Files	Coverage Δ	Complexity Δ
.../java/io/reactivex/observers/BaseTestConsumer.java	100% <100%> (ø)	116 <0> (ø)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.86% <0%> (-2.25%)	60% <0%> (ø)
...activex/internal/schedulers/ScheduledRunnable.java	98.07% <0%> (-1.93%)	29% <0%> (-1%)
...internal/operators/observable/ObservableCache.java	93.7% <0%> (-1.58%)	9% <0%> (ø)
...java/io/reactivex/processors/UnicastProcessor.java	98.8% <0%> (-1.2%)	67% <0%> (-1%)
.../operators/mixed/FlowableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	90.82% <0%> (-0.97%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableTimeoutTimed.java	98.37% <0%> (-0.82%)	3% <0%> (ø)
... and 20 more

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[bjoernQ]

Unfortunately #5258 didn't made it into 2.x as it seems

[vanniktech]

That explains it

[akarnokd]

Looks like it was never extended: https://github.com/JetBrains/intellij-community/blob/99614a63425774df58eea3ac92e2b20af1633663/plugins/junit_rt/src/com/intellij/junit4/ExpectedPatterns.java#L34

You don't need <> as this pattern matches expected: and but was: so you only need to replace Actual: .

[bjoernQ]

Thanks - will remove the (somewhat awkward <>) ... just added it to match the JUnit style but it's really not needed