Ensured that nested assertion scopes produce a nested context. So the statement

using var outerScope = new AssertionScope("outer");
using var innerScope = new AssertionScope("inner");

Results in a {context} as used in FailWith that contains outer/inner.

This should help #2597 produce a nicer message.

Also fixed an inconsistency in which one overload of the assertion scope constructor would not actually affect AssertionScope.Current.

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