tests/certs: rebuild certs with 2048-bit RSA keys

The previous test certificates contained RSA keys of only 1024 bits.
However, RSA claims that 1024-bit RSA keys are likely to become
crackable some time before 2010. The NIST recommends at least 2048-bit
keys for RSA for now.

Better use full 2048 also for testing.

Closes #2973