Previously, 2048-bit and 4096-bit RSA key files had their bitsize indicated in their filename, while the original
1024-bit keys hadn't. This commit unifies the naming scheme by always indicating the bitsize in the filename.
For uniformity, this commit adds tests for DER encoded, SHA1-2DES and SHA1-RC4-128-encrypted RSA keys; for SHA1-3DES encrypted keys, these were already present.
The first three test cases from test_suites_pkparse.data failed because
the key file they read requires DES to be read. However, MBEDTLS_DES_C
was missing from the dependency list.
