systemd-oomd: unit testable helper functions

Helpers used by the systemd-oomd daemon to read/fill in various contexts and
make detection and kill decisions.

i.e. a simplified/less configurable version of
https://github.com/facebookincubator/oomd/tree/master/src/oomd.

src/shared/tests.h

@@ -20,6 +20,7 @@ static inline bool manager_errno_skip_test(int r) {
 
 char* setup_fake_runtime_dir(void);
 int enter_cgroup_subroot(char **ret_cgroup);
+int enter_cgroup_root(char **ret_cgroup);
 int get_testdata_dir(const char *suffix, char **ret);
 const char* get_catalog_dir(void);
 bool slow_tests_enabled(void);