Fix SystemIOWindows.PathGetFullPath per review comments.
Add unit tests for SystemIOWindows.PathGetFullPath and SystemIOWindows.PrefixWithUNC. Also, fix a bug in the SystemIOWindows.PrefixWithUNC change that was exposed by these new unit tests.
In BackendToolTests, remove unused [Setup] and [TearDown] methods.
The handling of relative paths in SystemIOWindows was broken by changes for #4256.
Change SystemIOWindows.PrefixWithUNC to only prefix paths with `\\?\` when they can be prefixed; i.e., only prefix paths that are fully qualified and that don't contain relative path components like `.` or `..`. This way, relative paths passed to SystemIOWindows methods work correctly again. Also support network paths specified using forward slashes; e.g., `//example.com/share/foo.txt`.
Simplify SystemIOWindows.GetFullPath since Path.GetFullPath will honor `\\?\` prefixes and can replace forward slashes with backslashes, etc.
Copy and adapt source for Path.IsPathFullyQualified from https://github.com/dotnet/runtime, which is MIT licensed.
Add unit test for Duplicati.CommandLine.BackendTool to validate the fix.
This fixes#4632.
Kenneth Skovhede