Migrate tests to Kotlin, reorganise `java-tests`

[alexander-yevsyukov]

This PR migrates majority of tests into Kotlin and reorganises the project structure under java-tests.

Changes to tests in details

• Apart from conversion to Kotlin, tests were moved under the package io.spine.test with subpackages corresponding to a subsystem under the test.
• The test suite classes got the ITest suffix because they are integration tests.
• Custom assertions were consolidated under io.spine.validation.assertions.Assertions.kt.

Build changes under java-tests

• The modules were reorganised to consolidate majority of tests under the module validating.
• The module extra-definitions was renamed to consumer-dependency to clarify the meaning.
• Proto types to be validated and Java classes representing custom options and validating rules were moved under testFixtures to separate them from the test code.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 33.51%. Comparing base (53aaa8d) to head (a2232e2). Report is 116 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #143 +/- ## ============================================ - Coverage 34.21% 33.51% -0.71% + Complexity 360 356 -4 ============================================ Files 134 134 Lines 2709 2709 Branches 220 220 ============================================ - Hits 927 908 -19 - Misses 1719 1739 +20 + Partials 63 62 -1 ```