tap: inherit strict mode in subtests

Previous behaviour was quite illogical and unexpected. If user
once defined strict mode at start of tests it's expected that all
subtests also will have strict mode. However before this patch it
wasn't so. And wasn't clear at start why enabled strict mode
didn't work.
After this patch subtests strict mode will be the same as for
parent. This behaviour wasn't tested anyhow and wasn't documented.

Follow-up #4125

@TarantoolBot document
Title: clarify 'strict' behaviour in tap tests
Defined for root tap object strict mode will be the same for all
subtests.

Example:
```lua
t = require('tap').test('123')
t.strict = true

t:is_deeply({a = box.NULL}, {}) -- false

t:test('subtest', function(t)
    t:is_deeply({a = box.NULL}, {}) -- also false
end)
```

(cherry picked from commit 676582c4)