Reorganize helix files (#7353)

Directory.Build.targets

build/repo.targets

eng/helix/helix.proj

eng/helix/vstest/runtests.cmd

eng/helix/vstest/runtests.sh

eng/helix/xunit/runtests.cmd

...Build/test/Microsoft.AspNetCore.Blazor.Build.Tests.csproj

...crosoft.AspNetCore.DataProtection.Extensions.Tests.csproj

...ng/Hosting/test/Microsoft.AspNetCore.Hosting.Tests.csproj

...t.AspNetCore.ApiAuthorization.IdentityServer.Tests.csproj

...t/Identity.Test/Microsoft.AspNetCore.Identity.Test.csproj

...rel/test/Microsoft.AspNetCore.Server.Kestrel.Tests.csproj

...st/Interop.FunctionalTests/Interop.FunctionalTests.csproj

...rvers/Kestrel/test/Libuv.BindTests/Libuv.BindTests.csproj

...s/Kestrel/test/Sockets.BindTests/Sockets.BindTests.csproj

src/Shared/test/SkipOnHelixAttribute.cs

...osoft.AspNetCore.DeveloperCertificates.XPlat.Tests.csproj