Move tests related to Validation into the repository where they belong

[alexander-yevsyukov]

Code generation related to Validation used to be a part of McJava. Then we decided to have a separate library, which now resides in the dedicated repository.

We need to finish the separation and move most of the tests related to Validation into its code tree. It would make McJava build faster and its code cleaner. It would also help discovering issues with codegen related to Validation much sooner.

Here is the list of test suites that are currently disabled:

• io.spine.tools.mc.java.validation.gradle.ValidatingCodeGenTest
• io.spine.tools.mc.java.validation.gen.ValidationCodeGenTest
• io.spine.test.tools.validate.CustomConstraintsTest
• io.spine.test.tools.validate.ExternalConstraintTest
• io.spine.test.tools.validate.GoesConstraintTest

Classes with partially disabled tests:

• io.spine.test.tools.validate.RequiredConstraintTest

These tests must be re-enabled after they "land" into Validation, making sure their code is analyzed.
We do want most of these test pass with generated code (rather than runtime validation).

The only exception is tests of external validation constraints, which we do not want to support via a separate proto types, as we did in Spine v1.x.