Assert correct HttpRequestError in tests

[rzikm]

#88974 added HttpRequestError to programatically expose reason for failed Http requests, but we still have many tests which do not assert that correct enum member is selected, e.g.

runtime/src/libraries/System.Net.Http/tests/FunctionalTests/HttpClientHandlerTest.Headers.cs

Line 400 in 6de7549

await Assert.ThrowsAsync<HttpRequestException>(() => client.SendAsync(TestAsync, m));

Asserting expected HttpRequestError will also make sure we are consistent across HTTP versions.

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