Move workaround

Move the workaround to a static constructor so it runs early enough to affect the tests.

Author: martincostello

test/OpenTelemetry.Instrumentation.W3cTraceContext.Tests/W3CTraceContextTests.cs

@@ -41,9 +41,6 @@ public W3CTraceContextTests(ITestOutputHelper output)
     [System.Diagnostics.CodeAnalysis.SuppressMessage("Usage", "xUnit1026:Theory methods should use all of their parameters", Justification = "Need to use SkipUnlessEnvVarFoundTheory")]
     public async Task W3CTraceContextTestSuiteAsync(string value)
     {
-        // See https://learn.microsoft.com/en-us/dotnet/core/compatibility/core-libraries/10.0/default-trace-context-propagator
-        DistributedContextPropagator.Current = DistributedContextPropagator.CreatePreW3CPropagator();
-
         // configure SDK
         using var tracerProvider = Sdk.CreateTracerProviderBuilder()
             .AddAspNetCoreInstrumentation()