Adjust Is DSO Extension test check

Switch from a !file_exists() check to a !is_file() check.  is_file checks if a file exists and that it is a file, which file_exists checks that *something* exists, whether or not it's a file or directory.

My local setup has the following structure:

github
--hhvm
--hhvm-zmq

Previously, this check would see that I have a "../hhvm" folder and think we're running standard testing.  With this patch, it now ignores "../hhvm" as it's a directory and not a definate indicator we're inside the hhvm code base, and instead looks for the binary file hhvm, which is less likely to be a false-positive.