tests: Introduce afstest_GetProgname

tests: Introduce afstest_GetProgname

Currently, in tests/volser/vos-t.c we call afs_com_err as
"authname-t", which is clearly a mistake during some code refactoring
(introduced in commit 2ce3fdc5, "tests: Abstract out code to produce a
Ubik client").

We could just change this to "vos-t", but instead of specifying
constant strings everywhere, change this to figure out what the
current command is called, and just use that. Put this code into a new
function, afstest_GetProgname, and convert existing tests to use that
instead of hard-coding the program name given to afs_com_err.

Change-Id: I3ed02c89f93798568783c7d717e8fb2e39dcce14
Reviewed-on: https://gerrit.openafs.org/13991
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>