Add helper methods file_must_exist, file_wont_exist

Author: stomar

test/helper.rb

@@ -37,3 +37,13 @@ def linter_output
 
   stdout
 end
+
+def file_must_exist(filename)
+    assert File.exist?(filename),
+               "Expected file `#{filename}' to exist."
+end
+
+def file_wont_exist(filename)
+    assert !File.exist?(filename),
+               "Expected file `#{filename}' to not exist."
+end