testutils: Module-level method for reading test data

This patch moves the functions “_ReadTestData” and “_TestDataFilename”
to module level. They do not depend on a class instance at all.

Some cases of “utils.ReadFile(self._TestDataFilename(…))” are also
corrected.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>