Fix type of TestCase

[alxmtaylor]

Issue:
When running the conformance test suite from the CLI using ./fcs, the results are not output correctly if there are any failures alongside test passes

The fix:
TestCase in handlers.go should correspond to the TestCase in in result.go which is Fail []string json:"fail,omitempty"

How to test:
Run test suite using the CLI tool using your preferred config, where you know at least one test will fail, eg

./fcs run \   
    --filename dummy-v3.1.8.json \
    --config test-failing-config.json \
    --export export-config.json

Before this change you should see that the output of the above command would fail to output any failing test results. After this change you should see that failing test results are output correctly with the failure message along with any passes