selftests/bpf: Add an option for a debug shell in vmtest.sh

The newly introduced -s command line option starts an interactive shell.
If a command is specified, the shell is started after the command
finishes executing. It's useful to have a shell especially when
debugging failing tests or developing new tests.

Since the user may terminate the VM forcefully, an extra "sync" is added
after the execution of the command to persist any logs from the command
into the log file.

Signed-off-by: KP Singh <kpsingh@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210323014752.3198283-1-kpsingh@kernel.org