Support Jest --watchAll flag (#3804)

* Support Jest --watchAll flag

* Use clearer condition

* Add comma

Author: gaearon

packages/react-scripts/scripts/test.js

@@ -33,8 +33,12 @@ if (process.env.SKIP_PREFLIGHT_CHECK !== 'true') {
 const jest = require('jest');
 const argv = process.argv.slice(2);
 
-// Watch unless on CI or in coverage mode
-if (!process.env.CI && argv.indexOf('--coverage') < 0) {
+// Watch unless on CI, in coverage mode, or explicitly running all tests
+if (
+  !process.env.CI &&
+  argv.indexOf('--coverage') === -1 &&
+  argv.indexOf('--watchAll') === -1
+) {
   argv.push('--watch');
 }