test/system: Simplify running a subset of the tests with Bats >= 1.7.0

The 000-setup.bats and 999-teardown.bats files were added [1] at a time
when Bats didn't offer any hooks for suite-wide setup and teardown.

That changed in Bats 1.7.0, which introduced the setup_suite and
teardown_suite hooks.  These hooks make it easier to run a subset of the
tests, which is a good thing.

In the past, to run a subset of the tests, one had to do:
  $ bats ./test/system/000-setup.bats ./test/system/002-help.bats \
      ./test/system/999-teardown.bats

Now, one only has to do:
  $ bats ./test/system/002-help.bats

Commit e22a82f already added a dependency on Bats >= 1.7.0.
Therefore, it should be exploited wherever possible to simplify things.

[1] Commit 54a2ca1
    #751

[2] Bats commit fb467ec3f04e322a
    bats-core/bats-core#39
    https://bats-core.readthedocs.io/en/stable/writing-tests.html

test/system/000-setup.bats → test/system/setup_suite.bash

@@ -17,7 +17,10 @@
 
 load 'libs/helpers'
 
-@test "test suite: Set up" {
+setup_suite() {
+  bats_require_minimum_version 1.7.0
+  echo "# test suite: Set up" >&3
+
   local os_release="$(find_os_release)"
   local system_id="$(get_system_id)"
   local system_version="$(get_system_version)"
@@ -42,3 +45,14 @@ load 'libs/helpers'
 
   _setup_docker_registry
 }
+
+teardown_suite() {
+  bats_require_minimum_version 1.7.0
+  echo "# test suite: Tear down" >&3
+
+  _setup_environment
+
+  _clean_cached_images
+  _clean_docker_registry
+  _clean_temporary_storage
+}