Add a TEST option to toggle in-build testing as it was causing trouble when
the test build directory path length was longer than the maximum socket path
length. A workaround to this problem is noted in the Makefile. [1]

Prevent the GNUPG1 option and the TEST option from being enabled simultaneously
since the tests mainly revolve around the programs supplied with GnuPG 2.x.

Disable in-build tests for slave ports for now.

Move the flavor logic for the python slave port into the slave port Makefile
as it was not being evaluated correctly when in the master port Makefile.

Reported by: tijl (via private mail) [1]