fix: Fix unit test clean context (open-feature#313)

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## This PR
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- It fixes an issue where the context sometimes needs to be cleared
correctly.
- It should eliminate any further race conditions in the unit tests.

### Notes
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Following an investigation on xUnit shared contexts, I found we should
use `IDisposable` to clear data between tests. See
https://xunit.net/docs/shared-context for reference.

---------

Signed-off-by: André Silva <[email protected]>

Author: askpt

test/OpenFeature.Tests/ClearOpenFeatureInstanceFixture.cs

@@ -1,13 +1,14 @@
-namespace OpenFeature.Tests
+using System;
+
+namespace OpenFeature.Tests;
+
+public class ClearOpenFeatureInstanceFixture : IDisposable
 {
-    public class ClearOpenFeatureInstanceFixture
+    // Make sure the singleton is cleared between tests
+    public void Dispose()
     {
-        // Make sure the singleton is cleared between tests
-        public ClearOpenFeatureInstanceFixture()
-        {
-            Api.Instance.SetContext(null);
-            Api.Instance.ClearHooks();
-            Api.Instance.SetProviderAsync(new NoOpFeatureProvider()).Wait();
-        }
+        Api.Instance.SetContext(null);
+        Api.Instance.ClearHooks();
+        Api.Instance.SetProviderAsync(new NoOpFeatureProvider()).Wait();
     }
 }