Write guidance about checking in test data in test projects vs dotnet/runtime-assets

[layomia]

From @danmosemsft in https://github.com/dotnet/runtime/pull/42359#issuecomment-694490846:

Don't let it stop this PR, but I think these test strings of > 200KB probably should be in the test data repo as they just bloat the repo. Once you've merged them though, that bloat is locked in so it's more of a future suggestion.

From @ericstj in https://github.com/dotnet/runtime/pull/42359#issuecomment-694574240:

You can address the string suggestion in master. We're usually more sensitive about the type of data and how much it changes. Text data is typically OK at somewhat larger sizes if it doesn't change too often, since it compresses really well in git. In fact git uses zlib compression which is based on DEFLATE same as zip used in nupkg. Binary data, especially binary data that changes often, is what we make sure to put in assets packages. Also any large data that would be considered "optional" for some repo consumers. Maybe we could write some guidelines down somewhere :)

Todo:

• [ ] Write guidance about checking in test data in dotnet/runtime-assets vs in test projects
• [ ] Consider migrating JSON test payloads to dotnet/runtime-assets

[danmoseley]

I agree with Eric's guidance

[joperezr]

@layomia are you still planning on addressing this by 6.0? If not, can we adjust the milestone to future then?

[ericstj]

I don't think this meets the bar for 6.0 at this point, let's move it out.