HPCC-11321 - external file write/read test to regression suite

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #5787 from jakesmith/hpcc-11321

HPCC-11321 - external file write/read test to regression suite

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>