Fix assert_type failures when some nodes are deferred

[sobolevn]

Now it is quite the same as reveal_type. Which is defined here:

mypy/mypy/checkexpr.py

Line 4297 in 2c1fd97

if not self.chk.current_node_deferred:

Closes #15851

[AlexWaygood]

This LGTM! One minor suggestion to make the variable names in the test make a little more sense:

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

spack (https://github.com/spack/spack): typechecking got 1.29x slower (52.6s -> 67.7s)
(Performance measurements are based on a single noisy sample)