tests: Introduce libafstest_common

tests: Introduce libafstest_common

Currently, a few tests use the code in tests/common/ by linking
individual object files in there in addition to the test code (e.g.
linking ../common/config.o along with superuser-t.o).

This convention makes it very obnoxious to move code around in
tests/common/, since any users need to update their link lines. It
also makes it difficult for code in tests/common/ to make use of
functions in other tests/common/ files.

To fix this, just build all of the objects in tests/common/ into a
convenience library, called libafstest_common, and link the relevant
tests against that. Link a few requisite libraries (roken, rfc3961) in
libafstest_common, so each individual test doesn't need to link
against them.

Also link the TAP library itself in libafstest_common, so tests don't
have to explicitly link against it separately. To do this, convert it
into a libtool library, libafstest_tap.la.

Change-Id: I9c031c164efee20201336edcbfaff429e1d231b7
Reviewed-on: https://gerrit.openafs.org/14318
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>