Organize SendReceive tests and isolate non-parallel test collection

[antonfirsov]

Some SendReceive socket tests may be prone to timing issues on CI. This seems to be the root cause of #1712. We need a reliable way to run such tests to unblock the work on new UDP socket API-s in #33418.

This PR defines a new SendReceiveNonParallel test group, moving SendToRecvFrom_Datagram_UDP into that group. Since this is already a significant reorganization, it seemed reasonable to also:

• Harmonize naming: all SendReceive test classses are now named either SendReceive_[SubVariant]
  or SendReceiveNonParallel_[SubVariant]
• Split SendReceive.cs into multiple files:
  • SendReceive.cs for the parallel variants
  • SendReceiveNonParallel.cs for the new, non-parallel variants
  • Rename the non-generic class SendReceive to SendReceiveCommon (to avoid name collision and confusion with SendReceive<T>) and move it to SendReceiveCommon.cs
  • Move SendReceiveListener and SendReceiveUdpClient to separate files

Fixes #1712

/cc @geoffkizer

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Issue meta data

Issue content:
	Some `SendReceive` socket tests may be prone to timing issues on CI. This seems to be the root cause of #1712. We need a reliable way to run such tests to unblock the work on new UDP socket API-s in #33418. This PR defines a new SendReceiveNonParallel test group, moving SendToRecvFrom_Datagram_UDP into that group. Since this is already a significant reorganization, it seemed reasonable to also: Harmonize naming: all SendReceive test classses are now named either SendReceive_[SubVariant] or SendReceiveNonParallel_[SubVariant] Split SendReceive.cs into multiple files: SendReceive.cs for the parallel variants SendReceiveNonParallel.cs for the new, non-parallel variants Renamed the non-generic class SendReceive to SendReceiveCommon (to avoid name collision and confusion with SendReceive<T>) and moved it SendReceiveCommon.cs Moved SendReceiveListener and SendReceiveUdpClient to separate files Fixes #1712 /cc @geoffkizer </td>
Issue author:		antonfirsov
Assignees:	-
Labels:	`area-System.Net.Sockets` </td>
Milestone:	-

[geoffkizer]

Generally looks good, but I think we should change a couple class/file names... see above.

[antonfirsov]

@geoffkizer all addressed.

[antonfirsov]

The OuterLoop test failures are unrelated (System.Xml, Http and #44015).