Previously, mbedtls_pkcs5_pbes2() was unconditionally declared
in `pkcs5.h` but defined as a stub returning
`MBEDTLS_ERR_PKCS5_FEATURE_UNAVAILABLE` in case
MBEDTLS_ASN1_PARSE_C was not defined.
In line with the previous commits, this commit removes declaration
and definition from both `pkcs5.h` and `pkcs5.c` in case
MBEDTLS_ASN1_PARSE_C is not defined.
Add the `MBEDTLS_SELF_TEST` precompilation surrounding the self test functions,
which were missing this check in the header files. ( most of the header files were missing this check).
Addresses issue #971