SwiftDocCUtilitiesTests: adjust TestFileSystem's handling of paths (#655)

When constructing a `URL` with a file path, we should always use
`init(fileURLWithPath:)` as not all paths are represented as a URL and
require additional encoding.  On Windows, this would result in us
stripping the drive prefix (`C:`) which would corrupt the path and thus
fail to lookup the item.  In the second instance, this allowed us to
simply avoid an object copy entirely.

This helps improve the test pass rate significantly on Windows.

Author: compnerd

Tests/SwiftDocCUtilitiesTests/Utility/TestFileSystem.swift

@@ -209,12 +209,7 @@ class TestFileSystem: FileManagerProtocol, DocumentationWorkspaceDataProvider {
         filesLock.lock()
         defer { filesLock.unlock() }
 
-        guard let target = URL(string: path) else {
-            throw Errors.invalidPath(path)
-        }
-        
-        let parent = target.deletingLastPathComponent()
-        
+        let parent = URL(fileURLWithPath: path).deletingLastPathComponent()
         if parent.pathComponents.count > 1 {
             // If it's not the root folder, check if parents exist
             if createIntermediates == false {
@@ -262,8 +257,7 @@ class TestFileSystem: FileManagerProtocol, DocumentationWorkspaceDataProvider {
         filesLock.lock()
         defer { filesLock.unlock() }
 
-        guard let fileURL = URL(string: at.path),
-              files.keys.contains(fileURL.deletingLastPathComponent().path) else {
+        guard files.keys.contains(at.deletingLastPathComponent().path) else {
             throw NSError(domain: NSCocoaErrorDomain, code: NSFileNoSuchFileError, userInfo: [NSFilePathErrorKey: at.path])
         }