Add testing module as an internal project module

[ogaclejapan]

Use Robolectric for Android unit testing.
We added it to the project as a testing module so that we can write tests directly in the commonTest of each Kotlin Multiplatform module.

If you write a test by inheriting the UnitTest class as follows, RobolectricTestRunner will be used to run tests for Android.

import soil.testing.UnitTest

class FooTest extends UnitTest() {
  @Test
  fun test() { .. }
}