Get the mcsc handler tests to pass on Windows

autoload/ale/path.vim

@@ -9,14 +9,22 @@ endfunction
 
 " This function is mainly used for testing.
 " Simplify() a path, and change forward slashes to back slashes on Windows.
-function! ale#path#Winify(path) abort
-    let l:simplified_path = ale#path#Simplify(a:path)
+"
+" If an additional 'add_drive' argument is given, the current drive letter
+" will be prefixed to any absolute paths on Windows.
+function! ale#path#Winify(path, ...) abort
+    let l:new_path = ale#path#Simplify(a:path)
 
     if has('win32')
-        return substitute(l:simplified_path, '/', '\\', 'g')
+        let l:new_path = substitute(l:new_path, '/', '\\', 'g')
+
+        " Add a drive letter to \foo\bar paths, if needed.
+        if a:0 && a:1 is# 'add_drive' && l:new_path[:0] is# '\'
+            let l:new_path = fnamemodify('.', ':p')[:1] . l:new_path
+        endif
     endif
 
-    return l:simplified_path
+    return l:new_path
 endfunction
 
 " Given a buffer and a filename, find the nearest file by searching upwards

test/handler/test_mcsc_handler.vader

@@ -19,21 +19,21 @@ Execute(The mcs handler should handle cannot find symbol errors):
   \     'col' : 29,
   \     'text': 'error CS1001: ; expected',
   \     'type': 'E',
-  \     'filename': '/home/foo/project/bar/Test.cs'
+  \     'filename': ale#path#Winify('/home/foo/project/bar/Test.cs', 'add_drive'),
   \   },
   \   {
   \     'lnum': 101,
   \     'col': 0,
   \     'text': 'error CS1028: Unexpected processor directive (no #if for this #endif)',
   \     'type': 'E',
-  \     'filename': '/home/foo/project/bar/Test.cs'
+  \     'filename': ale#path#Winify('/home/foo/project/bar/Test.cs', 'add_drive'),
   \   },
   \   {
   \     'lnum': 10,
   \     'col': 12,
   \     'text': 'warning CS0123: some warning',
   \     'type': 'W',
-  \     'filename': '/home/foo/project/bar/Test.cs'
+  \     'filename': ale#path#Winify('/home/foo/project/bar/Test.cs', 'add_drive'),
   \   },
   \ ],
   \ ale_linters#cs#mcsc#Handle(347, [