tests/int: introduce the concept of unsafe tests

Some of runc integration tests may do something that I would not like
when running those on my development laptop. Examples include

 - changing the root mount propagation [1];
 - replacing /root/runc [2];
 - changing the file in /etc (see checkpoint.bats).

Yet it is totally fine to do all that in a throwaway CI environment,
or inside a Docker container.

Introduce a mechanism to skip specific "unsafe" tests unless an
environment variable, RUNC_ALLOW_UNSAFE_TESTS, is set. Use it
from a specific checkpoint/restore test which modifies
/etc/criu/default.conf.

[1]: https://github.com/opencontainers/runc/pull/5200
[2]: https://github.com/opencontainers/runc/pull/5207

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Dockerfile

@@ -84,6 +84,9 @@ RUN git config --global --add safe.directory /go/src/github.com/opencontainers/r
 
 WORKDIR /go/src/github.com/opencontainers/runc
 
+# Allow "unsafe" integration tests in a container.
+ENV RUNC_ALLOW_UNSAFE_TESTS=yes
+
 # Fixup for cgroup v2.
 COPY script/prepare-cgroup-v2.sh /
 ENTRYPOINT [ "/prepare-cgroup-v2.sh" ]