rsa: Use 2048 bit RSA keys for selftest.

* cipher/rsa.c (selftests_rsa): Use 2048 bit keys.
(selftest_encr_1024): Replaced by selftest_encr_2048.
(selftest_sign_1024): Replaced by selftest_sign_2048.
(selftest_encr_2048): Add check against known ciphertext.
(selftest_sign_2048): Add check against known signature.
(selftest_sign_2048): Free SIG_MPI.
* tests/pubkey.c (get_keys_new): Generate 2048 bit keys.

Use a 2048 bit keys for RSA selftest.
Check against the known signature/ciphertext after signing/encryption
in the selftests.
Also generate 2k keys in tests/pubkey.

Thanks to Ludwig Nussel.

Additional changes by wk:

• Reformat some strings and comments.
• Replace a free by xfree.
• Free SIG_MPI.
• Make two strings static.

• Signed-off-by: Werner Koch <wk@gnupg.org>