fix tests on windows

Author: lukaszsamson

apps/language_server/test/providers/formatting_test.exs

@@ -530,6 +530,7 @@ defmodule ElixirLS.LanguageServer.Providers.FormattingTest do
 
   defp format(path, project_dir) do
     project_dir = maybe_convert_path_separators(project_dir)
+    |> Path.absname()
     path = maybe_convert_path_separators("#{project_dir}/#{path}")
 
     source_file = %SourceFile{