In production on Python 3, ShellCheck would fail due to a string type

issue when trying to build replacement strings. This was missed in unit

tests due to an assumption being made that byte strings are appropriate

for diff content in "diff data" result from the API, when in fact those

must be Unicode strings, since the result is JSON (and encoding is

handled on the Review Board side).
Addressing this required fixing up the mismatch in the ShellCheck code,

and also updating the unit test helpers run_tool_execute() and

run_get_can_handle_file() to convert any sample bytestring content to

Unicode when building up a diff data payload.

Unit tests pass on all supported versions of Python.
Verified the ShellCheck fix with a customer.