test/repo: Add TestDataPath
test/repo: Add TestDataPath

This class represents the absolute path to a test data file.  This serves
two main purposes:
1. All test data files live in the same subdirectory, and this class
   encapsulates that commonality.
2. Canonically using an absolute path to access test data decouples the
   current working directory where a test is executed from the test result.

Signed-off-by: Andreas Kurth <62c4a7f75976b7a2fb8d79afddaccc3e9236aad5@lowrisc.org>

lowRISC/ot-sca,lowRISC/ot-sca