Make specs internal

Author: yevhenii-nadtochii

model/src/test/kotlin/io/spine/validation/GoesPolicySpec.kt

@@ -39,7 +39,7 @@ import org.junit.jupiter.params.ParameterizedTest
 import org.junit.jupiter.params.provider.MethodSource
 
 @DisplayName("`GoesPolicy` should reject the option")
-class GoesPolicySpec : AbstractCompilationTest() {
+internal class GoesPolicySpec : AbstractCompilationTest() {
 
     @MethodSource("io.spine.validation.GoesPolicyTestEnv#messagesWithUnsupportedTarget")
     @ParameterizedTest(name = "when target field type is `{0}`")

model/src/test/kotlin/io/spine/validation/PolicySpec.kt

@@ -65,7 +65,7 @@ import org.junit.jupiter.api.DisplayName
 import org.junit.jupiter.api.Test
 
 @DisplayName("Validation policies should")
-class PolicySpec {
+internal class PolicySpec {
 
     private lateinit var codegenContext: CodeGenerationContext
     private lateinit var blackBox: BlackBox