add option that allows developers to keep test data files locally instead of always downloading from FTP

[edwardhartnett]

We use some FTP files to test g2c, and I intend on adding more.

But downloading them every time I need them is slow. Instead, I will add an option that allows the builder of g2c to optionally specify a TEST_FILE_DIR. If that is specified, and if FTP_TEST_FILES is ON, then cmake will check for the files in TEST_FILE_DIR before downloading them. It will copy them if present, and use the FTP download if not.