test/system: Unbreak Podman's downstream Fedora CI

[debarshiray]

The paths to bats-assert and bats-support are broken, if bats(1) is invoked from any other location than the parent directory of the tests directory. eg., Podman's downstream Fedora CI invokes the tests as:

  $ cd /path/to/toolbox/test/system
  $ bats .

... and it led to [1]:

  1..306
  # test suite: Set up
  # Missing dependencies
  # Forgot to run 'git submodule init' and 'git submodule update' ?
  # test suite: Tear down
  not ok 1 setup_suite
  # (from function `setup_suite' in test file ./setup_suite.bash, line 33)
  #   `return 1' failed
  # bats warning: Executed 1 instead of expected 306 tests

Fallout from 2c0960660330dc6be6861502988695f9812c475a

[1] https://bugzilla.redhat.com/show_bug.cgi?id=2263968

[softwarefactory-project-zuul[bot]]

Build succeeded. https://softwarefactory-project.io/zuul/t/local/buildset/f93e4df8cf0f43efb7e7401a44709f44

:heavy_check_mark: unit-test SUCCESS in 4m 43s :heavy_check_mark: unit-test-migration-path-for-coreos-toolbox SUCCESS in 3m 22s :heavy_check_mark: unit-test-restricted SUCCESS in 3m 47s :heavy_check_mark: system-test-fedora-rawhide SUCCESS in 30m 46s :heavy_check_mark: system-test-fedora-39 SUCCESS in 31m 55s :heavy_check_mark: system-test-fedora-38 SUCCESS in 32m 22s