test: add test with a wrong behaviour of GC finalizer

`http.client` has an incorrect behaviour: HTTP-client object could be
GC-collected before GC-collecting an active chunked HTTP request. The
patch adds a test with an incorrect behaviour. Following commits will
fix this behaviour.

Needed for tarantool#9453

NO_CHANGELOG=testing
NO_DOC=testing

test/app-luatest/http_client_test.lua

@@ -1,3 +1,4 @@
+local popen = require('popen')
 local client = require('http.client')
 local json = require('json')
 local fiber = require('fiber')
@@ -1352,3 +1353,36 @@ g.test_http_client_io_post_parts = function(cg)
     t.assert_equals(io.status, 200, 'io')
     t.assert_equals(io.reason, 'Ok', '200 - Ok')
 end
+
+-- Read from ph:stderr and fail if it doesn't contain a `pattern'.
+local function grep_stderr_or_fail(ph, pattern)
+    local output = ''
+    t.helpers.retrying({}, function()
+        local chunk = ph:read({timeout = 0.05, stderr = true})
+        if chunk ~= nil then
+           output = output .. chunk
+        end
+        t.assert_str_contains(output, pattern)
+    end)
+end
+
+g.test_http_client_gc_finalizer_gh_9453 = function(cg)
+    local tarantool_exe = arg[-1]
+    local lua_code = [[local url = '%s'
+                       local c = require('http.client').new()
+                       local r = c:get(url, {chunked = true})
+
+                       c = nil
+                       collectgarbage()
+                       collectgarbage()
+
+                       r:read(1)]]
+    local ph = popen.new({ tarantool_exe, '-e', lua_code:format(cg.url) },
+                         { stderr = popen.opts.PIPE })
+    t.assert(ph)
+
+    -- Wait for the process startup.
+    grep_stderr_or_fail(ph, "IllegalParams: io: request must be io")
+
+    ph:close()
+end