Restrict test modifier chaining

[novemberborn]

Currently test modifiers allow silly and confusing combinations. We have linting rules to warn about them, and disallow some of them in the type definitions. We should go further and restrict the modifier chains so only useful combinations are allowed, but provide useful error feedback if disallowed combinations are used.

Examples of silly chains:

• test.always.afterEach.cb()
• test.after.test()
• test.always.always.serial.always.afterEach(() => {})

Examples of confusing chains:

• test.serial.afterEach() (test.serial.afterEach #1178)

See also the discussion in #1005.

[novemberborn]

test.serial.afterEach() (#1178)

Whilst this one is potentially confusing, I don't want to stop people from using the import {serial as test} from 'ava' shorthand. It's not removed in #1670.