Generate stable test output

Currently the output of the tests is not stable when running in different environments:
* some absolute file paths are printed, which depend on the path of the build directory
* `Last updated` timestamp is printed in the output of many tests

This PR removes the above glitches. It is a prerequisite for fixing issue #30

Details of the changes:
* florentx@1c49ffe patches the `time.localtime` function and the `TZ` environment variable which are used to generate the `Last updated` timestamp
* florentx@b3fb9cc adds a stub `barchart.py` file to avoid printing an error message with the absolute path of the missing file
* florentx@f4c5f46 replaces absolute path `{docfile}` with relative path `{docname}.txt`