tests: add support for tests expected to fail

Add a new macro FAIL_TEST that can be used to define tests that are
supposed to fail. To distinguish the supposed outcome of a test, add a
field to 'struct test'.

However, simply adding a field to 'struct test' will make all tests past
the first one in an executable to be garbage. Apparently, the variables
of type 'struct test' have different alignment when put into a special
section than otherwise, and the compiler will get the skip from one
'struct test' to the next wrong.

Explicitly specify the alingment of 'struct test' to be 16 bytes, which
is what it seems to be in the special section on x86_64.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>