System tests: honor $OCI_RUNTIME (for CI)

Some CI systems set $OCI_RUNTIME as a way to override the
default crun. Integration (e2e) tests honor this, but system
tests were not aware of the convention; this means we haven't
been testing system tests with runc, which means RHEL gating
tests are now failing.

The proper solution would be to edit containers.conf on CI
systems. Sorry, that would involve too much CI-VM work.
Instead, this PR detects $OCI_RUNTIME and creates a dummy
containers.conf file using that runtime.

Add: various skips for tests that don't work with runc.

Refactor: add a helper function so we don't need to do
the complicated 'podman info blah blah .OCIRuntime.blah'
thing in many places.

BUG: we leave a tmp file behind on exit.

Signed-off-by: Ed Santiago <santiago@redhat.com>

test/system/260-sdnotify.bats

@@ -17,9 +17,9 @@ function setup() {
 
     # sdnotify fails with runc 1.0.0-3-dev2 on Ubuntu. Let's just
     # assume that we work only with crun, nothing else.
-    run_podman info --format '{{ .Host.OCIRuntime.Name }}'
-    if [[ "$output" != "crun" ]]; then
-        skip "this test only works with crun, not '$output'"
+    runtime=$(podman_runtime)
+    if [[ "$runtime" != "crun" ]]; then
+        skip "this test only works with crun, not $runtime"
     fi
 
     basic_setup