Data in tests_file is inconsistent

[jscook2345]

Note that in some records, the value field is a dictionary.

https://qa-reports.linaro.org/api/testruns/2549893/tests_file/

In others it is a string.

https://qa-reports.linaro.org/api/testruns/2549854/tests_file/

I think this should be changed to always be a dictionary. Ex:

{"build/build_process": {"result":"pass"}

I'm not sure if this would break current api dependencies. What do you think?