tests: Configure the environments to use scdaemon from build tree.

* tests/gpgme/gpgme-defs.scm: Use the scdaemon from the build tree
when writing a 'gpg-agent.conf'.
* tests/gpgsm/gpgsm-defs.scm: Likewise.
* tests/openpgp/defs.scm: Likewise.

As of 97a2394ecafaa6f58e4a1f70ecfd04408dc15606 gpg may query the
scdaemon for a signing key to use. To make sure that the agent calls
the right scdaemon, we provide the path explicitly in the
'gpg-agent.conf' that is used in the tests, similar to what we do for
the agent itself and the pinentry.

• GnuPG-bug-id: T3165
• Signed-off-by: Justus Winter <justus@g10code.com>