testType: support defer testing

[unional]

currently, testType.* can be used in test directly. e.g.:

it('blah', () => {
  testType.equal<A, B>(true)
}

However, when the tests get complicates or when there are may duplication, you would like to extract part of the test and reuse it:

it('blah', () => {
  testMyType(...)
}

it('bruh', () => {
  testMyType(...)
}

testMyType(...) {
  testType....
}

Since testType.* are 💥 immediate, they can't be used this way.

Not sure if there is a way to achieve this a the library level as this is crossing the type-value boundary (i.e. fail type throws Error)