InputDecorator M3 test migration step2 (#143369)

## Description

This PR is the second step for the M3 test migration for `InputDecorator` (step 1 was https://github.com/flutter/flutter/pull/142981).

This PR migrate the two first tests of the M2 section. Those were big tests. I splitted them in several testsn organized in groups, and I narrowed their scope when possible.

@justinmc  I did not move yet the M2 tests to a separate file (I move them to a group) because it would mean we loss the line history which is useful during the migration. In the next step, I will focus on moving out some tests that are in the 'Material2' group (the ones that are ok with both M2 and M3).

## Related Issue

Related to https://github.com/flutter/flutter/issues/139076

## Tests

Adds several tests for M3.