[ReactNative] Fix 32 bit check compile error

Summary:
@public

The test file is getting compiled when hitting cmd+R which fails for 64-bit
devices even if not trying to run tests.  Change back to runtime check to fix.

Test Plan:
cmd+R works for iPhone 6, cmd+U fails as expected on iPhone 6, works
for iPhone 5

Examples/UIExplorer/UIExplorerIntegrationTests/IntegrationTestsTests.m

@@ -26,7 +26,7 @@
 - (void)setUp
 {
 #if __LP64__
-  #error Tests should be run on 32-bit device simulators (e.g. iPhone 5)
+  RCTAssert(false, @"Tests should be run on 32-bit device simulators (e.g. iPhone 5)");
 #endif
 
   NSOperatingSystemVersion version = [[NSProcessInfo processInfo] operatingSystemVersion];