Fix asan.test_format_exception test. NFC (#16397)

This test was recently added but was failing under `asan` because
it was leaking the exception objects.

tests/test_core.py

@@ -1545,7 +1545,7 @@ def test_exceptions_rethrow_missing(self):
 
   def test_format_exception(self):
     self.set_setting('DISABLE_EXCEPTION_CATCHING', 0)
-    self.set_setting('DEFAULT_LIBRARY_FUNCS_TO_INCLUDE', ['$formatException'])
+    self.set_setting('DEFAULT_LIBRARY_FUNCS_TO_INCLUDE', ['$formatException', '__cxa_decrement_exception_refcount', '__cxa_increment_exception_refcount'])
     self.set_setting('EXPORTED_FUNCTIONS', ['_main', 'formatException', '_emscripten_format_exception', '_free'])
     self.maybe_closure()
     src = '''
@@ -1582,7 +1582,13 @@ class myexception : public exception {
               try {
                   Module["_throw_exc"](i);
               } catch(p) {
+                  // Because we are catching and handling the exception in JS, the normal
+                  // exception catching C++ code doesn't kick in, so we need to make sure we free
+                  // the exception, if necessary. By incrementing and decrementing the refcount
+                  // we trigger the free'ing of the exception if its refcount was zero.
+                  ___cxa_increment_exception_refcount(p);
                   console.log(Module["formatException"](p).replace(/0x[0-9a-f]*/, "xxx"));
+                  ___cxa_decrement_exception_refcount(p);
               }
             }
           });