OSS: Fix $ENTRY_FILE check for non-Debug Xcode builds

Summary:
The original $ENTRY_FILE check was added in https://github.com/facebook/react-native/pull/29012 to help catch misconfiguration for the entry JS file. That turned out breaking some RNTester builds/tests, so https://github.com/facebook/react-native/pull/29263 was added to accommodate the fact that RNTester .xcodeproj file has its own directory hierarchy.

The 2nd PR had multiple issues:
* It is incorrect to assume that the $ENTRY_FILE always exists in the parent dir of the .xcodeproj location. This caused an issue in RC 0.66: https://github.com/react-native-community/releases/issues/249#issue-983474535
* RNTester has since moved to packages/rn-tester/ (from RNTester/), hence breaking that assumption

It turns out RNTester .xcodeproj has incorrectly misconfigured this JS bundling step (not sure since when). The original script invocation passed in the correct path for `RNTesterApp.ios.js`, but as an arg to the `react-native-xcode.sh` instead of by setting `ENTRY_FILE` env var.

So this diff does 2 things:
* Undid https://github.com/facebook/react-native/pull/29263
* Fix RNTester JS bundling invocation to set the ENTRY_FILE correctly

{F659123377}

Changelog: [iOS][Fixed] Unbreak $ENTRY_FILE handling for JS bundling

Reviewed By: lunaleaps

Differential Revision: D30690900

fbshipit-source-id: 7c5802b3eac56c0456edcd4b7478bfa4af48fc27

scripts/react-native-xcode.sh

@@ -74,7 +74,7 @@ else
   ENTRY_FILE=${1:-index.js}
 fi
 
-if [[ $DEV != true && ! -f "../$ENTRY_FILE" ]]; then
+if [[ $DEV != true && ! -f "$ENTRY_FILE" ]]; then
   echo "error: Entry file $ENTRY_FILE does not exist. If you use another file as your entry point, pass ENTRY_FILE=myindex.js" >&2
   exit 2
 fi