Fixup ProcessAsync_AwaitersUseTheResultOfExecutor (#21274)
Fixup ProcessAsync_AwaitersUseTheResultOfExecutor (#21274)

This test simulates concurrent calls on a CacheTagHelper to ensure only one enters a critical section.
However it re-uses a TagHelperOutput instance that's modified concurrently which isn't valid or what happens at runtime.

Fixes https://github.com/dotnet/aspnetcore/issues/20825
aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore