test_runner: remove quotes for coverageColors properties

Author: dmnsgn

lib/internal/test_runner/utils.js

@@ -36,10 +36,10 @@ const {
 const { compose } = require('stream');
 
 const coverageColors = {
-  '__proto__': null,
-  'high': green,
-  'medium': yellow,
-  'low': red,
+  __proto__: null,
+  high: green,
+  medium: yellow,
+  low: red,
 };
 
 const kMultipleCallbackInvocations = 'multipleCallbackInvocations';