OpenSSL::Test: Treat SRCDATA directory specially, as it might not exist

Not all tests come with a SRCDATA directory.  if it doesn't exist, we
simply drop it from the internal table of directories.

OpenSSL::Test::srcdata_dir() and OpenSSL::Test::srcdata_file() may
return undef in that case.  However, recipes shouldn't try to refer to
a non-existing data directory, so if that happens, it's a programming
error and must be corrected.

Fixes #15679

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15700)