Manual Test CancelAsyncConnections intermittently fails in worrying ways.

### Describe the bug
Running manual tests locally I've been seeing intermittent failures on an unusual test. I was seeing it on an experimental branch but has now reproduced it on clean main branch.

I've seen this fail, be incredibly slow (30 mins+) or fail with an access violation from the native sni. This is the most likely failure mode.

To run tests I do a full artifacts clean, build and then run tests.

```
[xUnit.net 00:08:43.51] ---> System.Component
Failed Microsoft.Data.SqlClient.ManualTesting.Tests.AsyncCancelledConnectionsTest.CancelAsyncConnections [38 s]
Error Message:
Assert.Empty() Failure
Collection: ["System.InvalidOperationException: Internal connect"..., "Microsoft.Data.SqlClient.SqlException (0x80131904)"..., "Microsoft.Data.SqlClient.SqlException (0x80131904)"..., "Microsoft.Data.SqlClient.SqlException (0x80131904)"..., "Microsoft.Data.SqlClient.SqlException (0x80131904)"...]
Stack Trace:
at Microsoft.Data.SqlClient.ManualTesting.Tests.AsyncCancelledConnectionsTest.RunCancelAsyncConnections(SqlConnectionStringBuilder connectionStringBuilder) in E:\Programming\csharp7\SqlClient\src\Microsoft.Data.SqlClient\tests\ManualTests\SQL\AsyncTest\AsyncCancelledConnectionsTest.cs:line 0
at Microsoft.Data.SqlClient.ManualTesting.Tests.AsyncCancelledConnectionsTest.CancelAsyncConnections() in E:\Programming\csharp7\SqlClient\src\Microsoft.Data.SqlClient\tests\ManualTests\SQL\AsyncTest\AsyncCancelledConnectionsTest.cs:line 32
Standard Output Messages:
00:00:05.0020806 True Started:98 Done:85 InFlight:13 RowsRead:197598 ResultRead:1662 PoisonedEnded:40 nonPoisonedExceptions:469 PoisonedCleanupExceptions:0 Count:4 Found:0
00:00:10.0005771 True Started:100 Done:98 InFlight:2 RowsRead:263405 ResultRead:2215 PoisonedEnded:53 nonPoisonedExceptions:469 PoisonedCleanupExceptions:0 Count:4 Found:0
00:00:15.0012357 True Started:100 Done:98 InFlight:2 RowsRead:263405 ResultRead:2215 PoisonedEnded:53 nonPoisonedExceptions:469 PoisonedCleanupExceptions:0 Count:4 Found:0
00:00:20.0051765 True Started:100 Done:98 InFlight:2 RowsRead:263405 ResultRead:2215 PoisonedEnded:53 nonPoisonedExceptions:469 PoisonedCleanupExceptions:0 Count:4 Found:0
00:00:25.0089728 True Started:100 Done:98 InFlight:2 RowsRead:263405 ResultRead:2215 PoisonedEnded:53 nonPoisonedExceptions:469 PoisonedCleanupExceptions:0 Count:4 Found:0
00:00:30.0035748 True Started:100 Done:98 InFlight:2 RowsRead:263405 ResultRead:2215 PoisonedEnded:53 nonPoisonedExceptions:469 PoisonedCleanupExceptions:0 Count:4 Found:0
00:00:34.9992757 True Started:100 Done:98 InFlight:2 RowsRead:263405 ResultRead:2215 PoisonedEnded:53 nonPoisonedExceptions:469 PoisonedCleanupExceptions:0 Count:4 Found:0
00:00:38.6946916 True Started:100 Done:100 InFlight:0 RowsRead:269593 ResultRead:2267 PoisonedEnded:53 nonPoisonedExceptions:471 PoisonedCleanupExceptions:0 Count:5 Found:0
System.InvalidOperationException: Internal connection fatal error.
at Microsoft.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySi
Microsoft.Data.SqlClient.SqlException (0x80131904): A transport-level error has occurred when receiving results from the server. (provider: SSL Provider, error: 0 - The specified data could not be dec
Microsoft.Data.SqlClient.SqlException (0x80131904): A connection was successfully established with the server, but then an error occurred during the login process. (provider: TCP Provider, error: 0 -
Microsoft.Data.SqlClient.SqlException (0x80131904): A transport-level error has occurred when receiving results from the server. (provider: TCP Provider, error: 0 - The file name is too long.)
--->
Microsoft.Data.SqlClient.SqlException (0x80131904): Execution Timeout Expired. The timeout period elapsed prior to completion of the operation or the server is not responding.
---> System.Component
```

### To reproduce
Run the manual test suite run or possibly just the `Microsoft.Data.SqlClient.ManualTesting.Tests.AsyncCancelledConnectionsTest.RunCancelAsyncConnections` repeatedly and it will eventually fail.

### Expected behavior
The test should reliably pass. It having intermittent failures and especially one as serious as an access violation is worrying for reliability.

### Further technical details
Microsoft.Data.SqlClient version: (4.0.0 main)
.NET target: netcore 3.1
SQL Server version: SQL Server 2017
Operating system: Windows 10, native SNI