Change way Task Cancel CAT is implemented to get around 'Cannot cancel a Failed task' error.
Change way Task Cancel CAT is implemented to get around 'Cannot cancel a Failed task' error.

- Previously we sent a loop of cancel commands and read their output
- The output string is overwritten every time we send a cancel command
- Eventually this meant our output string contained the error instead of the expected FailureReason
- Now we cancel once and read repeatedly, preventing CC from raising the 'Cannot cancel a Failed task' error.

[#111473134]

Signed-off-by: Frank Kotsianas <7292b1604e9876e8e1934a101736830424483797@pivotal.io>

matt-royal/cf-acceptance-tests,matt-royal/cf-acceptance-tests,matt-royal/cf-acceptance-tests,matt-royal/cf-acceptance-tests,matt-royal/cf-acceptance-tests,matt-royal/cf-acceptance-tests,matt-royal/cf-acceptance-tests,matt-royal/cf-acceptance-tests