Fix up System.Text.Encodings.Web test namespace to be consistent with other corefx projects

[ahsonkhan]

See dotnet/corefx#39900 (comment)

Rename the namespace in the test project from Microsoft.Framework.WebEncoders to System.Text.Encodings.Web.Tests

https://github.com/dotnet/corefx/blob/c6f5ceedea28edf01ae7d3e13c02935669dad434/src/System.Text.Encodings.Web/tests/IHtmlEncoder.cs#L8

cc @GrabYourPitchforks

[igor-susic]

@ahsonkhan This seems like a good start to get to know codebase a little and make the first contribution, if it is ok with you I would like to take this one.

[ahsonkhan]

Awesome, go for it. I can't yet assign this to you, so I'll assign it to myself (as a placeholder).

@karelz, can we get @igor-susic added as a contributor?

[igor-susic]

@ahsonkhan No problem, I'm on it 👍