Use SimpleCov.add_filter block instead of string

Using a string is almost always a bad idea, because if any part of
the path contains the string, it will filter out the path. Switch
to using a block and doing a match on the entire path prefix.

Author: jeremyevans

test/coverage_helper.rb

@@ -3,7 +3,7 @@
 
   SimpleCov.start do
     enable_coverage :branch
-    add_filter "/test/"
+    add_filter{|f| f.filename.start_with?(__dir__ + "/")}
     add_group('Missing'){|src| src.covered_percent < 100}
     add_group('Covered'){|src| src.covered_percent == 100}
   end