[wasm] Add assembly level skip for System.Net.Requests on Browser

[mdh1418]

When running the tests for System.Net.Requests on Browser, the tests that hang all have async Task. Until there is support for these tests, an assembly level skip will be added to skip the entire test suite. This PR looks to make the System.Net.Requests.Tests test suite pass on WebAssembly.
Tests run: 0, Errors: 0, Failures: 0, Skipped: 0. Time: 0.003914s

Contributes to #38422

The following is the git diff --stat showing the corresponding changes if hanging tests were disabled instead of the entire suite.

src/libraries/System.Net.Requests/tests/FileWebRequestTest.cs                      |  2 +-
src/libraries/System.Net.Requests/tests/HttpWebRequestTest.cs                      | 52 ++++++++++++++++++++++++++--------------------------
src/libraries/System.Net.Requests/tests/HttpWebResponseHeaderTest.cs               |  6 +++---
src/libraries/System.Net.Requests/tests/HttpWebResponseTest.cs                     | 12 ++++++------
src/libraries/System.Net.Requests/tests/RequestStreamTest.cs                       | 40 ++++++++++++++++++++--------------------

[ghost]

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[MaximLipnin]

System.Net.Request is modified to throw PNSE at assembly level on Browser WASM in #40274.