Don't Overwrite Already Existing Environment Vars

Closes #1181

If a variable is already set via `putenv` or if the variable is already
present in the `$_ENV` array, it will no longer be ovewritten by
`PHPUnit_Util_Configuration::handlePHPConfiguration`.

This allows users to pass in environment variables fia the command line
in the usual ways:

    bash$ SOME_VAR="a value" ./path/to/phpunit