HPCC-11092 - TOPN in child query not renewing limit per iteration

Limit was set at activity initialization time, should have been
set at start()
Also changed over to using a CThorExpandingRowArray and cleared
up the code. Add binaryInsert to CThorExpandingRowArray.
Move topn.ecl test to new regression suite.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>