Fix ExceptionCounter.oneThread test case

Summary: Test fails because of loop unrolling: frames are not being grouped up

Reviewed By: ot

Differential Revision: D4034910

fbshipit-source-id: 159d3a4f116f90fdd0d51ee9de38634167ed0dc4

folly/experimental/exception_tracer/test/ExceptionCounterTest.cpp

@@ -45,7 +45,9 @@ void throwAndCatch(F f) {
 
 TEST(ExceptionCounter, oneThread) {
   throwAndCatch(foo);
-  for (int i = 0; i < 10; ++i) {
+
+  // Use volatile to prevent loop unrolling (it screws up stack frame grouping).
+  for (volatile int i = 0; i < 10; ++i) {
     throwAndCatch(bar);
   }