Replace shell variable with file

Debug output was being truncated always at about the same point, which
turned out it happens because of some limit to environment variables
size imposed by bash when it's run in as /bin/sh. Since our debug output
is huge it make much more sense to store it in a file in WORKDIR.
Fix agent debug log filename.

For debug runs, agent debug log filename was garbled since test_name
contains full path. The debug log is now simply named agent-debug.log.
Keep agent output in the stdout stream...

...when spawning an agent in cf-serverd acceptance tests, it's needed
for the testsuite to scan for "Pass" or "FAIL" keyword.
Re-enable all cf-serverd tests.