Use stronger path-matching for simplecov filters

The filter for spec/ was actually matching basically everything in the
gem (and discarding it), because it's all in lib/rspec/, and simplecov
is doing a string-based infix match. I'm not certain when that changed,
but my guess is 2018, in this PR:

    simplecov-ruby/simplecov#616

I also adjusted the minimum_coverage down to 99, since we're currently
at 99.26, and not 100%. I'll get on that shortly.

script/rspec_with_simplecov

@@ -28,10 +28,11 @@ begin
     require 'simplecov'
 
     SimpleCov.start do
-      add_filter "bundle/"
-      add_filter "tmp/"
-      add_filter "spec/"
-      minimum_coverage(100)
+      root File.expand_path("../..", __FILE__)
+      add_filter %r{/bundle/}
+      add_filter %r{/tmp/}
+      add_filter %r{/spec/}
+      minimum_coverage(99)
     end
   end
 rescue LoadError # rubocop:disable Lint/SuppressedException