[testing] Marbles in error messages of tests

Feature Request - Marbles in testing

Is your feature request related to a problem? Please describe.
When working on operators in the RxJS source you need to parse errors like this:

     AssertionError: 
Expected 
	{"frame":10,"notification":{"kind":"N","value":"a","hasValue":true}}
	{"frame":100,"notification":{"kind":"N","value":"b","hasValue":true}}
	{"frame":160,"notification":{"kind":"N","value":"c","hasValue":true}}
	{"frame":210,"notification":{"kind":"C","hasValue":false}}
	
to deep equal 
	{"frame":10,"notification":{"kind":"N","value":"y","hasValue":true}}
	{"frame":100,"notification":{"kind":"N","value":"b","hasValue":true}}
	{"frame":160,"notification":{"kind":"N","value":"c","hasValue":true}}
	{"frame":210,"notification":{"kind":"C","hasValue":false}}

These errors are hard to read/understand.

Describe the solution you'd like
I would like to propose replacing or amending the messages with marbles.

E.g.

     AssertionError: 
Expected 
	-a--------b-----c----C		
to deep equal 
	-y--------b-----c----C	

I've already implemented a POC here.

My question is: do the developers of RxJS want this? If so, are there any requests/objections/comments.

Review the linked proof-of-concept branch for its marble-based error output and compare it with the current RxJS test failures described in the issue. Before starting implementation, confirm with the maintainers whether this behavior is wanted and what output and test coverage would define completion.