[corlib] Improve file:// url handling in AppDomainSetup. (#16161)
* [corlib] Improve file:// url handling in AppDomainSetup.
WiX 3 installer toolkit uses file:/// prefix when setting ApplicationBase for newly created domain.
Currently this leads to invalid path exception:
file:///C:\... -> /C:\... -> full path expansion with current workdir on different drive -> Y:\C:\...
Later it fails check for multiple ':' -> NotSupportedException ("The given path's format is not supported.").
This patch uses logic already present in AppDomain::Load().
* [Test] Add a test for file:/// application base.
* [Test] Fix test failure on Windows.
* [corlib] Simplify file: protocol check for AppDomainSetup.