Add cron and the python 3 mock packages to the Bullseye slave image

When using dpkg with the --root flag to install a deb package in a
chroot, dpkg checks to see if the users and groups specified in the
/var/lib/dpkg/statoverride file are valid. The problem is, it checks
against the host system's /etc/passwd and /etc/group files, instead of
/etc/passwd and /etc/group inside the chroot.

In the slave image for Buster, cron appears to have been automatically
installed, and so the crontab group existed. This doesn't appear to be
true for Bullseye, so explicitly install it. This makes sure that the
crontab group exists on the slave image itself.

Also install the mock python 3 package, this is needed by some test.

Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>

sonic-slave-bullseye/Dockerfile.j2

@@ -72,6 +72,7 @@ RUN apt-get update && apt-get install -y \
         libzmq5 \
         libzmq3-dev \
         jq \
+        cron \
 # For quagga build
         libreadline-dev \
         texlive-latex-base \
@@ -398,7 +399,7 @@ RUN pip3 install setuptools==49.6.00
 RUN pip3 install wheel==0.35.1
 
 # For building sonic-utilities
-RUN pip3 install fastentrypoints
+RUN pip3 install fastentrypoints mock
 
 # For running Python unit tests
 RUN pip3 install pytest-runner==5.2