Avoid running generate-ftpindex during tests

Avoid running generate-ftpindex during tests

The attempt to rebuild the FTP server index is normally harmless, since most
test machines will not actually have /usr/local/bin/generate-ftpindex and the
error from the inability to execute that tool is ignored anyway.

However, if the testsuite is ever actually run on the FTP upload server, this
will prevent a large amount of spurious work rebuilding the FTP indexes.