Add SafeMode option to test settings for Excel launch

[novitk]

Introduce SafeMode property to ExcelFactAttribute, ExcelTestSettingsAttribute, allowing tests to specify launching Excel in safe mode (/safe). This basically allows to run tests in working area that avoids conflicts with other addins.

[Sergey-Vlasov]

What about adding a more general string ExcelArguments (?) option instead of bool SafeMode?

And add an additional example test instead of modifying existing FunctionTestNET6().

[novitk]

What about adding a more general string ExcelArguments (?) option instead of bool SafeMode?
was thinking about it, but decided against it. Three reason: don't see much practical usage, extra complexity(a need for a dictionary per string), explicitness(people may not be aware of safety mode to begin with).

And add an additional example test instead of modifying existing FunctionTestNET6()
Actually both tests don't run together for me on master. The box pops up with:
ExampleAddinNET8 Add-In - ExcelDna.Host
The required .NET Desktop Runtime 8.0.0 x64 is incompatible with the runtime 10.0.3 already loaded in the process.
You can try to disable other Excel add-ins to resolve the conflict.
as /safe runs its own excel instance it was a cheap way to make both instances work.

Feel free to modify or drop in whatever way you'd like though. I moved away from using this project to reduce dependencies and complexity - just spinning Excel directly in xunit fixture was simple enough.