Split tests to avoid duplicate names.

tests/root.bats

+_test_root () {
+    name="${1:-$BATS_TEST_DESCRIPTION}"
+    tag="${2:-latest}"
+    docker build -t "runas-$name" --build-arg "image=$name:$tag" ./
+    docker run --rm -v "$BATS_TEST_DIRNAME:/data" "runas-$name" touch "$name.root"
+    test "$(id -u)" = "$(stat -c '%u' $name.root)"
+    test "$(id -g)" = "$(stat -c '%g' $name.root)"
+}
+
+teardown () {
+    docker image ls --format '{{.ID}}' runas-* | xargs -r docker image rm
+    rm -f *.touch
+}
+
+@test "ubuntu" { _test_root buildpack-deps bionic; }
+@test "debian" { _test_root buildpack-deps stretch; }
+@test "centos" { _test_root; }
+@test "fedora" { _test_root; }
+@test "alpine" { _test_root; }

tests/test.bats → tests/user.bats

-_test_root () {
-    name="${1:-$BATS_TEST_DESCRIPTION}"
-    tag="${2:-latest}"
-    docker build -t "runas-$name" --build-arg "image=$name:$tag" ./
-    docker run --rm -v "$BATS_TEST_DIRNAME:/data" "runas-$name" touch "$name.root"
-    test "$(id -u)" = "$(stat -c '%u' $name.root)"
-    test "$(id -g)" = "$(stat -c '%g' $name.root)"
-}
-
 _test_user () {
     name="${1:-$BATS_TEST_DESCRIPTION}"
     tag="${2:-latest}"
@@ -23,11 +14,6 @@ teardown () {
     rm -f *.touch
 }
 
-@test "ubuntu" { _test_root buildpack-deps bionic; }
-@test "debian" { _test_root buildpack-deps stretch; }
-@test "centos" { _test_root; }
-@test "fedora" { _test_root; }
-@test "alpine" { _test_root; }
 @test "ubuntu" { _test_user buildpack-deps bionic; }
 @test "debian" { _test_user buildpack-deps stretch; }
 @test "centos" { _test_user; }