Fix JUnit report location when running Jest

Summary:
We were incorrectly writing jest's junit output to ~/reports/ instead of ~/react-native/reports.

Run on Circle and confirm JUnit test results are rendered: https://circleci.com/gh/hramos/react-native/2208

[INTERNAL][MINOR][CI] - JUnit test collection

[skip ci]
Closes https://github.com/facebook/react-native/pull/19349

Differential Revision: D8062654

Pulled By: hramos

fbshipit-source-id: 72066270042dfae8afce62469fcfabb57bd405a6

package.json

@@ -123,7 +123,7 @@
   ],
   "scripts": {
     "test": "jest",
-    "test-ci": "JEST_JUNIT_OUTPUT=\"~/reports/junit/js-test-results.xml\" jest --maxWorkers=2 --ci --testResultsProcessor=\"jest-junit\"",
+    "test-ci": "JEST_JUNIT_OUTPUT=\"reports/junit/js-test-results.xml\" jest --maxWorkers=2 --ci --testResultsProcessor=\"jest-junit\"",
     "flow": "flow",
     "lint": "eslint .",
     "prettier": "find . -name node_modules -prune -or -name '*.js' -print | xargs prettier --write",