restore mistakenly removed iffs in their explicit form

Due to confusion arisen from "iff" standing for "if and only if",
this commit uses the latter.

googletest/test/gtest_environment_test.cc

@@ -116,7 +116,7 @@ void Check(bool condition, const char* msg) {
   }
 }
 
-// Runs the tests.  Return true if successful.
+// Runs the tests.  Return true if and only if successful.
 //
 // The 'failure' parameter specifies the type of failure that should
 // be generated by the global set-up.