tests: Generalize temp dir management

tests: Generalize temp dir management

Currently, afstest_BuildTestConfig calls afstest_mkdtemp (our thin
wrapper around mkdtemp) to create its temporary config dir. We may
want to make new tests, though, that create a temp dir for other
purposes. To make that easier, move a little more code into
afstest_mkdtemp, so the caller doesn't need to construct the template.

To allow callers to clean up such temporary dirs, change
afstest_UnlinkTestConfig into a more general function,
afstest_rmdtemp. Allow this new function to remove all files in a dir,
not just files one-level-deep. To avoid needing to write our own
traversal and removal logic, just run 'rm -rf' via a new function,
afstest_systemlp().

Move these temp dir-related functions from config.c into files.c,
since they are no longer specific to config dirs.

Change-Id: I16750a2f30e98c9ca2e14dfb7d3fc9bc5d456e8d
Reviewed-on: https://gerrit.openafs.org/14632
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>