Fix websocket partial message receive test (dotnet/corefx#29826)

Fix websocket partial message receive test (dotnet/corefx#29826)

This test was originally disabled before we switched from WinHTTP to a managed
implementation on Windows. And any other failure noted on Linux was probably due
to a transient network/environment issue.

I simplified the test to use smaller buffer sizes which might help with any test
environment issues. These updated sizes still meet the requirements of what needs
to be tested. If we find that this test is still failing frequently then we will
switch to a loopback echo websocket server (which we don't have fully implemented yet).

Fixes dotnet/corefx#9296

Commit migrated from https://github.com/dotnet/corefx/commit/077280bd5b20d158136ea000bb6c4c86ec40ac0c