Revert to xunit's default parallel algorithm in Polly.Core.Tests

[martincostello]

xunit 2.8.0+ changes the default algorithm use to run async tests in parallel: docs

This change caused an existing test to fail when dependabot upgraded the project's version of xunit in #2186. This is the specific test that failed:

Polly/test/Polly.Core.Tests/Hedging/HedgingStrategyOptionsTests.cs

Line 22 in 957444f

[InlineData(true)]

Having asked over in the xunit repository (see xunit/xunit#3024), a workaround was made to revert back to the previous default algorithm to fix the test and ingest the dependency update: 35967a3

While this fixes the CI and allows us to stay up-to-date with xunit, it isn't the optimal solution.

Instead we should determine how to refactor the test to assert on the required intent (that the hedged action executes on a different thread) of the test so that we can revert the changes in the commit referenced above and instead use xunit's default parallel algorithm (option 3 in the linked discussion).