Turn StyleCop off in test projects

`StyleCop` is checking if a source file has the license header.
There might be exceptions defined in it and our generated source code in `Generated` directory does not trigger the error.

But in the mean time, the customization code of one library should also be considered as part of the generated library which I think do not have to have a license header.
But styleCop requires us must have a license header for them which might be inconvenient.

how to reproduce:
1. we had license header in [this file](https://github.com/microsoft/typespec/blob/1c5d61fcfb563efd1cd14cf3bf67a0a4031905d3/packages/http-client-csharp/generator/TestProjects/Local/Unbranded-TypeSpec/src/Custom/Friend.cs#L1), remove the license header.
2. run `dotnet build .\generator\TestProjects\Local\Unbranded-TypeSpec\UnbrandedTypeSpec.sln` we would see the following `styleCop` errors:
```
C:\Users\dapzhang\Documents\workspace\typespec\packages\http-client-csharp\generator\TestProjects\Local\Unbr
anded-TypeSpec\src\Custom\Friend.cs(1,1): error SA1636: The file header copyright text should match the copy
right text from the settings. (https://github.com/DotNetAnalyzers/StyleCopAnalyzers/blob/master/documentatio
n/SA1636.md) [C:\Users\dapzhang\Documents\workspace\typespec\packages\http-client-csharp\generator\TestProje
cts\Local\Unbranded-TypeSpec\src\UnbrandedTypeSpec.csproj]
```

It is fine that we regard this as by design, if so, please just close this issue.