test(mergeWith): include mergeWith tests from 7.x

[jakovljevic-mladen]

Description:
Since PR #7281 that was recently merged to 7.x branch had some tests that are helpful for current master branch, I wanted to cherry pick that commit and create a PR for it, but I failed due to fix being done in non-existing file on master branch tree. So I picked it on my own because mergeWith operator on master does not have this issue at all.

I also removed two tests from mergeWith spec file that are related to mergeAll operator. They are redundant and already exist in mergeAll spec file:

rxjs/spec/operators/mergeAll-spec.ts

Lines 529 to 557 in 2947583

	it('should merge two observables', (done) => {
	const a = of(1, 2, 3);
	const b = of(4, 5, 6, 7, 8);
	const r = [1, 2, 3, 4, 5, 6, 7, 8];
	of(a, b)
	.pipe(mergeAll())
	.subscribe({
	next: (val) => {
	expect(val).to.equal(r.shift());
	},
	complete: done,
	});
	});
	it('should merge two immediately-scheduled observables', (done) => {
	const a = scheduled([1, 2, 3], queueScheduler);
	const b = scheduled([4, 5, 6, 7, 8], queueScheduler);
	const r = [1, 2, 4, 3, 5, 6, 7, 8];
	scheduled([a, b], queueScheduler)
	.pipe(mergeAll())
	.subscribe({
	next: (val) => {
	expect(val).to.equal(r.shift());
	},
	complete: done,
	});
	});

Related issue (if exists):
PR #7281