Remove usage of -parallel flag on dotnet-test

[natemcmaster]

dotnet-test no longer supports -parallel. Any tests that require non-parallel test runs should configure this via code instead.

Example:

[assembly: CollectionBehavior(DisableTestParallelization = true)]

cref https://xunit.github.io/docs/running-tests-in-parallel.html cref https://xunit.github.io/docs/configuring-with-xml

cc @bricelam (EF Core SQL Server tests) cc @pranavkm we should be able to remove this from aspnetci config

[bricelam]

Filed an issue to react in EF.

[natemcmaster]

cc @cesarbs