Use "Legacy Build System"

Summary:
React Native needs to be updated to support XCode 10 (GitHub issue: [19573](https://github.com/facebook/react-nåative/issues/19573)). Until we make that change, our CI needs to rely on the legacy build system in XCode 10 to build `RNTester`, as opposed to the new build system made default in XCode 10. This diff uses the `-UseModernBuildSystem=NO` `xcodebuild` flag to enforce that requirement.

**Note:** I did a search within `react-native-github/scripts` for `xcodebuild` and passed in the `-UseModernBuildSystem=NO` flag. I figured that every time we use `xcodebuild`, we should enforce the build system.

� This should hopefully fix the `react-native-oss-ios` test.

Reviewed By: hramos

Differential Revision: D10456139

fbshipit-source-id: 795f32b3ceba6a5a24ab6ccbf4f5160d42746801

Author: RSNara

scripts/objc-test.sh

@@ -83,6 +83,7 @@ xcodebuild \
   -scheme $SCHEME \
   -sdk $SDK \
   -destination "$DESTINATION" \
+  -UseModernBuildSystem=NO \
   build test \
   | xcpretty --report junit --output "$HOME/react-native/reports/junit/$TEST_NAME/results.xml"
 
@@ -93,6 +94,7 @@ xcodebuild \
   -project "RNTester/RNTester.xcodeproj" \
   -scheme $SCHEME \
   -sdk $SDK \
+  -UseModernBuildSystem=NO \
   build
 
 fi

scripts/run-ci-e2e-tests.js

@@ -216,6 +216,7 @@ try {
             `"${scheme}"`,
             '-sdk',
             sdk,
+            '-UseModernBuildSystem=NO',
             'test',
           ].join(' ') +
             ' | ' +