…unset) override by default (#151357)

While exploring #107607, I noticed that flutter_tools test results change based on whether `dart test` is run from a terminal or from a process (such as a Dart program). I also ran into this while writing tests for #150667.

This is due to tests that rely on the global `Stdio` instance, on which the `hasTerminal` property depends on whether the tool is being invoked from a terminal.

Ideally, `testUsingContext` would require any tests that depend on `globals.stdio` to define an override for `Stdio`, but this is not the case. Until a solution to this more general problem is figured out, I think we should have `testUsingContext` always provide a `Stdio` override by default.