Remove test_main
Remove test_main

This test has been disabled for over two years now
(31417e798403687fb893abc27685c09d475c178d). Let's remove it.

The functionality in this test should be covered through end-to-end
tests invoking the command line.

olofk/fusesoc,lowRISC/fusesoc,olofk/fusesoc,lowRISC/fusesoc