Fix regexp for filtering out non-root results being set too early

If user configures a custom `SimpleCov.root`, results outside of the new
root wouldn't be properly filtered out since the regexp for filtering
was being set at require time, before the `SimpleCov.root` configuration
had been applied.

Instead, lazily set this regexp, so that by the time it is used, the new
`SimpleCov.root` value is already setup.

lib/simplecov/useless_results_remover.rb

@@ -5,12 +5,14 @@ module SimpleCov
   # Select the files that related to working scope directory of SimpleCov
   #
   module UselessResultsRemover
-    ROOT_REGX = /\A#{Regexp.escape(SimpleCov.root + File::SEPARATOR)}/io.freeze
-
     def self.call(coverage_result)
       coverage_result.select do |path, _coverage|
-        path =~ ROOT_REGX
+        path =~ root_regx
       end
     end
+
+    def self.root_regx
+      @root_regx ||= /\A#{Regexp.escape(SimpleCov.root + File::SEPARATOR)}/i.freeze
+    end
   end
 end

spec/useless_results_remover_spec.rb

@@ -32,4 +32,25 @@
     expect(subject).to have_key(source_path)
     expect(subject[source_path]["lines"]).to be_kind_of(Array)
   end
+
+  context "when changing coverage root" do
+    around do |example|
+      begin
+        previous_root_regx = SimpleCov::UselessResultsRemover.instance_variable_get(:@root_regx)
+        SimpleCov::UselessResultsRemover.instance_variable_set(:@root_regx, nil)
+        previous_root = SimpleCov.root
+
+        SimpleCov.root "lib"
+
+        example.run
+      ensure
+        SimpleCov.root previous_root
+        SimpleCov::UselessResultsRemover.instance_variable_set(:@root_regx, previous_root_regx)
+      end
+    end
+
+    it "ignores stuff outside of the new coverage root" do
+      expect(subject).to be_empty
+    end
+  end
 end