Update JsonWriter performance tests and add more unit tests.

[ahsonkhan]

[WIP] Depends on #2292 (where we split the types) for CI to be green.

BenchmarkDotNet=v0.10.14.534-nightly, OS=Windows 10.0.16299.431 (1709/FallCreatorsUpdate/Redstone3)
Intel Xeon CPU E5-1620 v2 3.70GHz, 1 CPU, 8 logical and 4 physical cores
Frequency=3604598 Hz, Resolution=277.4234 ns, Timer=TSC
.NET Core SDK=2.1.300-rtm-008838
  [Host]     : .NET Core 2.1.0-rtm-26514-02 (CoreCLR 4.6.26514.02, CoreFX 4.6.26514.02), 64bit RyuJIT
  DefaultJob : .NET Core 2.1.0-rtm-26514-02 (CoreCLR 4.6.26514.02, CoreFX 4.6.26514.02), 64bit RyuJIT

Method	IsUTF8Encoded	Formatted	Mean	Error	StdDev	Median	Gen 0	Allocated
WriterSystemTextJsonBasic	False	False	13,007.23 ns	206.9702 ns	193.6001 ns	13,007.52 ns	-	0 B
WriterNewtonsoftBasic	False	False	24,385.41 ns	323.5019 ns	302.6039 ns	24,460.31 ns	0.0610	472 B
WriterSystemTextJsonHelloWorld	False	False	79.65 ns	0.1798 ns	0.1501 ns	79.68 ns	-	0 B
WriterNewtonsoftHelloWorld	False	False	252.18 ns	2.8943 ns	2.2597 ns	251.23 ns	0.0591	312 B
WriterSystemTextJsonBasic	False	True	15,455.75 ns	27.5224 ns	22.9825 ns	15,455.74 ns	-	0 B
WriterNewtonsoftBasic	False	True	38,726.82 ns	423.6548 ns	353.7709 ns	38,878.64 ns	0.0610	640 B
WriterSystemTextJsonHelloWorld	False	True	85.28 ns	1.5050 ns	1.4078 ns	86.04 ns	-	0 B
WriterNewtonsoftHelloWorld	False	True	351.49 ns	6.7202 ns	6.9011 ns	356.92 ns	0.0911	480 B
WriterSystemTextJsonBasic	True	False	13,229.05 ns	18.3385 ns	14.3175 ns	13,228.26 ns	-	0 B
WriterNewtonsoftBasic	True	False	24,424.51 ns	237.3358 ns	210.3920 ns	24,457.86 ns	0.0610	416 B
WriterSystemTextJsonHelloWorld	True	False	141.67 ns	2.8499 ns	2.7990 ns	143.30 ns	-	0 B
WriterNewtonsoftHelloWorld	True	False	286.15 ns	4.9989 ns	4.6760 ns	284.86 ns	0.0486	256 B
WriterSystemTextJsonBasic	True	True	15,354.43 ns	178.5350 ns	158.2666 ns	15,431.49 ns	-	0 B
WriterNewtonsoftBasic	True	True	37,463.68 ns	97.7627 ns	86.6641 ns	37,465.13 ns	0.0610	584 B
WriterSystemTextJsonHelloWorld	True	True	145.69 ns	0.3185 ns	0.2659 ns	145.73 ns	-	0 B
WriterNewtonsoftHelloWorld	True	True	384.19 ns	6.8438 ns	6.4017 ns	387.05 ns	0.0806	424 B

[adamsitnik]

LGTM!

I saw the that the unit tests are called "basic" and you don't check for edge case support.

I assume that you plan to do this in the future, so please don't forget to check some edge cases (like a string containing ",", nested json etc). I wonder how handling such edge cases is going to affect the performance.

[adamsitnik]

Could you please move the duplicated code to a helper methd?

TextWriter writer;
if (IsUTF8Encoded)
{
	_memoryStream.Seek(0, SeekOrigin.Begin);
	writer = _streamWriter;
}
else
{
	_stringBuilder.Clear();
	writer = new StringWriter(_stringBuilder);
}

[ahsonkhan]

Good call, will do.

[adamsitnik]

good idea to create Random with constant seed! 👍

[ahsonkhan]

@dotnet-bot test this please

[ahsonkhan]

@dotnet-bot test this please

[ahsonkhan]

@dotnet-bot test Innerloop Ubuntu16.04 Debug Build and Test