stream, test: add test for _readableState.readingMore

[mcollina]

Part of #8644.

stream._readableState.readingMore

node/lib/_stream_readable.js

Lines 91 to 92 in 774146d

	// if true, a maybeReadMore has been scheduled
	this.readingMore = false;

cc @Fishrock123 @nodejs/streams

[mansimarkaur]

I'd like to work on this as my first contribution.

[chmln]

@mansimarkaur OK if I contribute?

[mansimarkaur]

@chmln sure.