There are apparently some dependency conditions between ncdump/tst_nccopy4.sh and ncdump/tst_netcdf4.sh.
Fixed by creating a per-test directory into which all created intermediates are stored. Also created a test_ncdump.sh file to hold common code.

## Other Changes
* Remove some uses of strlcat from plugin-related code because strlcat is not always defined on some platforms and the plugin code generally should not rely on accessing libnetcdf.
* Cleanup the handling of test plugins.