Fixed exportVariable multiple backslash problem

When a global variable contains an object which contains a string which contains multiple backslashes (phew!) and you are using `@runTestsInSeparateProcesses` and `@preserveGlobalState`, `PHPUnit_Util_GlobalState` modifies the result of `serialize` in a way that breaks the `unserialize`.

This gist contains the smallest demonstration I could cook up of what happens: https://gist.github.com/shabbyrobe/cf0b2fd76220ecd6e991

I wrote a test case which showed that all cases except the multiple backslash object one passed, then replaced the call to `str_repace` with a call to `var_export` which fixes the problem without breaking anything I could think of.