Certstore system tests fail if certificates are not installed.

[FAlbertDev]

After building Botan on Windows, the Botan certstore system tests (src/tests/test_certstor_system.cpp) failed on my system. The root certificate (D-TRUST Root Class 3 CA 2 EV 2009) was somehow not installed on my system, which caused this test to fail. The test implicitly asserts that some well-known CA certificates are installed. There are various options for how to handle this issue:

1. We can ignore it and hope my system was the only one where the respective certificate was not installed. Maybe we could at least add a comment to the test that it fails if the certificate is not found on the system.
2. We skip the test if the certificate is not found instead of failing it.
3. Same as 2, but we add a flag to botan-test to avoid that we skip this test in CI.

In my opinion, it's not very urgent since no one has complained so far. It could be frustrating for a user when installing Botan, though.