toEmitValue matchers

[MillerSvt]

I suggest adding a few matchers for a narrower stream check.

const expected = cold('a', {a: value});
expect(stream).toBeObservable(value);

// replaces with

expect(stream).toEmitValueFirst(value);

const expected = cold('--a', {a: value});
expect(stream).toBeObservable(value);

// replaces with

expect(stream).toEmitValueNth(value, 3);

// there are no alternatives

expect(stream).toEmitValueLast(value);