test: fix common.mustCall

Don't run the 'has function been called?' checks if the test is exiting
with an error because a failed check will mask the real exception.

v0.8 doesn't have the _fatalException machinery in src/node.js and
src/node.cc so it doesn't have this issue.

test/common.js

@@ -163,7 +163,9 @@ process.on('exit', function() {
 var mustCallChecks = [];
 
 
-function runCallChecks() {
+function runCallChecks(exitCode) {
+  if (exitCode !== 0) return;
+
   var failed = mustCallChecks.filter(function(context) {
     return context.actual !== context.expected;
   });