Make the nearest file search result absolute paths, and add tests to cover it.

autoload/ale/util.vim

@@ -33,7 +33,15 @@ endfunction
 " Given a buffer and a filename, find the nearest file by searching upwards
 " through the paths relative to the given buffer.
 function! ale#util#FindNearestFile(buffer, filename) abort
-    return findfile(a:filename, fnamemodify(bufname(a:buffer), ':p') . ';')
+    let l:buffer_filename = fnamemodify(bufname(a:buffer), ':p')
+
+    let l:relative_path = findfile(a:filename, l:buffer_filename . ';')
+
+    if !empty(l:relative_path)
+        return fnamemodify(l:relative_path, ':p')
+    endif
+
+    return ''
 endfunction
 
 function! ale#util#GetFunction(string_or_ref) abort

test/test_nearest_file_search.vader

@@ -0,0 +1,11 @@
+Execute(Open a file some directory down):
+  silent! cd /testplugin/test
+  :e! top/middle/bottom/dummy.txt
+
+Then(We should be able to find a configuration file further up):
+  AssertEqual expand('%:p:h:h:h:h') . '/top/example.ini', ale#util#FindNearestFile(bufnr('%'), 'example.ini')
+
+Execute(Do nothing):
+
+Then(We shouldn't find anything for files which don't match):
+  AssertEqual '', ale#util#FindNearestFile(bufnr('%'), 'cantfindthis')