tests: Fix Python detection.

* tests/gpgme/gpgme-defs.scm (python): Fix Python detection.

In 25e6444b3f4601c7821beab06bc4520deacb007b we changed the way GPGME
creates the build directory for the Python bindings. We now use the
basename of the interpreter as an identifier, that means we have to
get the base name right, a link to the same binary is not sufficient.

• Signed-off-by: Justus Winter <justus@g10code.com>