tc-testing: Add newline when writing test case files

When using the -i feature to generate random ID numbers for test
cases in tdc, the function that writes the JSON to file doesn't
add a newline character to the end of the file, so we have to
add our own.

Signed-off-by: Lucas Bates <lucasb@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>