Make assert_covers check array items for coverage

Treating arrays as maps in `assert_covers` is error-prone so we fixed it
in commit 95a1c3f ("Fix `assert_covers` treat arrays as maps"), but
we did it over-zealously - now arrays are treated just like scalars
(numbers, for example). This breaks the case when array items are
supposed to be checked for coverage for example:

```lua
t.assert_covers({
    {a = 1, b = 10},
    {a = 2, b = 20},
    {a = 3, b = 30},
}, {{a = 1}, {a = 2}, {a = 3}})
```

Fix this.

Follow-up #405
Closes #439

Author: locker

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## Unreleased
+
+- Fixed a bug when `assert_covers` didn't check array items for coverage and
+  instead treated arrays as scalars (gh-439).
+
 ## 1.3.0
 
 - Fixed a bug when `assert_covers` treats arrays as maps (gh-405).

luatest/assertions.lua

@@ -443,12 +443,12 @@ local function table_slice(actual, expected)
     if type(expected) ~= 'table' or type(actual) ~= 'table' then
         return actual
     end
-
+    local sliced
     if utils.table_is_array(actual) or utils.table_is_array(expected) then
-        return actual
+        sliced = table.copy(actual)
+    else
+        sliced = {}
     end
-
-    local sliced = {}
     for k, _ in pairs(expected) do
         sliced[k] = table_slice(actual[k], expected[k])
     end

test/luatest_test.lua

@@ -113,6 +113,8 @@ g.test_assert_covers = function()
     subject({a = box.NULL}, {a = box.NULL})
     subject({a = box.tuple.new({1})}, {a = box.tuple.new({1})})
     subject({1, 2}, {1, 2})
+    subject({{a = 1, b = 10}, {a = 2, b = 20}, {a = 3, b = 30}}, {{a = 1}, {a = 2}, {a = 3}})
+    subject({x = {{a = 1, b = 10}, {a = 2, b = 20}}, y = {}}, {x = {{a = 1}, {a = 2}}})
     subject({[0] = 'zero', [1] = 'one'}, {[0] = 'zero'})
 
     helper.assert_failure(subject, {a = 1, b = 2, c = 3}, {a = 2})
@@ -123,6 +125,9 @@ g.test_assert_covers = function()
     helper.assert_failure(subject, {a = {b = 1, c = 2}}, {a = {b = 1, c = 2, d = 3}})
     helper.assert_failure(subject, {1, 2}, {})
     helper.assert_failure(subject, {1, 2, 3}, {1, 2})
+    helper.assert_failure(subject, {{a = 1, b = 10}, {a = 2, b = 20}}, {{a = 1}})
+    helper.assert_failure(subject, {{a = 1, b = 10}, {a = 2, b = 20}}, {{a = 1}, {a = 2}, {a = 3}})
+    helper.assert_failure(subject, {1, 2}, {nil, 2})
     helper.assert_failure(subject, {1, 2, foo = 'bar'}, {1, 2})
     helper.assert_failure(subject, {a = {1, 2, 3}}, {a = {1, 2}})
     helper.assert_failure(subject, {a = {1, 2}}, {a = {}})