Add Testcases for time-stamping functionality

This commit adds testcases for the new time-stamping functionality. To trigger
these testcases, the environment variables $GIT_TEST_TSA_URL and
$GIT_TEST_TSA_CAPATH have to be set. They do not default to any value to avoid
spamming a pre-defined Time Stamping Authority (TSA).
As testing the RFC3161 functionality without a trusted time-stamp signature is
not possible, all tests are disabled by default.

Signed-off-by: Anton Würfel <anton.wuerfel@fau.de>
Signed-off-by: Phillip Raffeck <phillip.raffeck@fau.de>