Re-enable dual-mode socket tests that were disabled on Linux\Mac

[steveharter]

We should re-enable tests that were disabled on Linux\Mac. The tests would randomly fail because of the way that port allocation is different than on Windows. On Linux\Mac, when asking for an available free port, the returned port number is a random unused port. On Windows, it is incremental and eventually rolls over.

These tests are negative tests that connect to a V4 port and then send\listen on a V6 port (or vise-versa) of the same number expecting an unused port and thus failure case. However, in Linux\Mac there is a chance that the port is active instead of unused, causing the test to succeed, and\or interfering with a 'good' test. The port may be active on Windows too, causing similar problems, but the odds are much less as any given test run should not have active ports open that longs.

Two options to re-enable these tests:

1. Enable a test mode where a given test is flagged an runs serially
2. Move the tests to a new assembly with the CollectionBehavior attribute (DisableTestParallelization=true, MaxParallelThreads=1)

For these to work, all other tests should be done running, including tests in other assemblies (at least those tests which use sockets).

An alternative way to re-enable these tests is to keep them in the current assembly, but change the logic to something like this to preserve the original intent of making sure the socket is isn't 'dual mode':

1. Bind to address family with port 0 so we get unused port
2. Bind to the other address family + same port
3. The bind should succeed (otherwise retry by going back to step 1)
4. Unbind (so we can do the receive on it)
5. Do the actual receive on that unbound port
6. Should fail (as these tests are negative tests)

This alternative approach will increase reliability but there is a small chance between 4 and 5 another test grabbed that port.
However, if 3 succeeds then it probably proves that the socket wasn't dual mode because you can't bind to the same port + family twice; thus perhaps the test can stop there and not include 4-6.

See issues dotnet/corefx#16548, dotnet/corefx#9017, dotnet/corefx#13213, dotnet/corefx#17362, dotnet/corefx#17154, dotnet/corefx#13967

Tests that were disabled below, located in DualModeSocketTest.cs. Note history should be checked for each of the commits as there are cases of Linux vs. Mac differences in the original test code which needs to be accounted for when the tests are enabled again.

https://github.com/dotnet/corefx/blob/master/src/System.Net.Sockets/tests/FunctionalTests/DualModeSocketTest.cs

DualModeConnect_IPAddressListToHost_Throws
BeginAcceptV6BoundToSpecificV4_CantConnect
BeginAcceptV4BoundToSpecificV6_CantConnect
BeginAcceptV6BoundToAnyV4_CantConnect
AcceptAsyncV6BoundToSpecificV4_CantConnect
AcceptAsyncV4BoundToSpecificV6_CantConnect
AcceptAsyncV6BoundToAnyV4_CantConnect
SendToV4IPEndPointToV6Host_NotReceived
SendToV6IPEndPointToV4Host_NotReceived
SendToAsyncV4IPEndPointToV6Host_NotReceived
SendToAsyncV6IPEndPointToV4Host_NotReceived
ReceiveMessageFromV6BoundToAnyV4_NotReceived
ReceiveFromV6BoundToSpecificV4_NotReceived
ReceiveFromV4BoundToSpecificV6_NotReceived
ReceiveFromV6BoundToAnyV4_NotReceived
BeginReceiveFromV6BoundToSpecificV4_NotReceived
BeginReceiveFromV4BoundToSpecificV6_NotReceived
BeginReceiveFromV6BoundToAnyV4_NotReceived
ReceiveMessageFromV6BoundToSpecificV4_NotReceived
ReceiveMessageFromV4BoundToSpecificV6_NotReceived

[karelz]

These tests are skipped on Linux/Mac, we should revisit them for 2.1.

[antonfirsov]

Related: #1712

Third option: introduce centralized logic to distribute non-ephemeral ports to test cases in a thread-safe way.