luatest hangs if tests perform too many checks without yielding execution

[locker]

Add the following test to test/bad_test.lua:

local t = require('luatest')
local g = t.group()

for i = 1, 100 do
    g['test_' .. i] = function()
        for j = 1, 100 do
            t.assert_equals(i, i)
            t.assert_equals(j, j)
        end
    end
end

Run it with disabled capturing (-c):

bin/luatest -c -v test/bad_test.lua

It hangs:

$ bin/luatest -c -v test/bad_test.lua
Tarantool version is 3.4.0-entrypoint-227-gf1ccb2f142f6
Started on Fri Apr  4 17:56:30 2025
luatest | 2025-04-04 17:56:30.081 [412780] main/104/luatest/luatest.log runner.lua:306 I> Bootstrap finished: 1 test(s), 0 group(s)
    bad.test_1 ...
    bad.test_1 ... (0.005s) Ok
    bad.test_69 ...
    bad.test_69 ... (0.004s) Ok
    bad.test_7 ...

Notes:

• If capturing is enabled, the test passes successfully.
• If logs are written to the unified log file (-l <path>), the test passes as well.
• The issue was reproduced at commit 6c5c8b7. At the previous commit (2f16ac7), the test passes.

[locker]

Actually, the bug may occur even with unified logging - if the same test case performs too many checks:

local t = require('luatest')
local g = t.group()

g.test_foo = function()
    for i  = 1, 10000 do
        t.assert_equals(i, i)
    end
end

It didn't happen before commit 6c5c8b7 because we configured Tarantool to log through a pipe, which works in the non-blocking mode by default. Now, we log directly to /dev/%d/fd, which is still a pipe, but perceived by Tarantool as a file.