Remove unused KotlinSourceSetImpl.defaultIsTestModule; simplify logic of test modules detection

^KT-37127

idea/kotlin-gradle-tooling/src/KotlinMPPGradleModelBuilder.kt

@@ -785,16 +785,7 @@ class KotlinMPPGradleModelBuilder : ModelBuilderService {
                 }
             }
 
-            // TODO DISCUSS AT REVIEW: can we simplify it to:
-            //      sourceSet.isTestModule = importingContext.compilationsBySourceSet(sourceSet)?.all { it.isTestModule} ?: false
-            //                                                                                                           ^
-            //                                                                                                           |
-            //                                                                                                          sic!
-            //  The difference from the current behaviour would be in case if source set is not included into any compilation AND
-            //  KotlinSourceSetImpl.defaultIsTestModule is set to 'true'. Though I don't know who and why would do that.
-            importingContext.compilationsBySourceSet(sourceSet)?.all { it.isTestModule }?.let { isTest ->
-                sourceSet.isTestModule = isTest
-            }
+            sourceSet.isTestModule = importingContext.compilationsBySourceSet(sourceSet)?.all { it.isTestModule } ?: false
 
             importingContext.compilationsBySourceSet(sourceSet)?.let { compilations ->
                 val platforms = compilations.map { it.platform }

idea/kotlin-gradle-tooling/src/KotlinMPPGradleModelImpl.kt

@@ -36,7 +36,6 @@ class KotlinSourceSetImpl(
     override val dependencies: Array<KotlinDependencyId>,
     override val dependsOnSourceSets: Set<String>,
     val defaultPlatform: KotlinPlatformContainerImpl = KotlinPlatformContainerImpl(),
-    val defaultIsTestModule: Boolean = false
 ) : KotlinSourceSet {
 
     constructor(kotlinSourceSet: KotlinSourceSet, cloningCache: MutableMap<Any, Any>) : this(
@@ -46,14 +45,15 @@ class KotlinSourceSetImpl(
         HashSet(kotlinSourceSet.resourceDirs),
         kotlinSourceSet.dependencies,
         HashSet(kotlinSourceSet.dependsOnSourceSets),
-        KotlinPlatformContainerImpl(kotlinSourceSet.actualPlatforms),
-        kotlinSourceSet.isTestModule
-    )
+        KotlinPlatformContainerImpl(kotlinSourceSet.actualPlatforms)
+    ) {
+        this.isTestModule = kotlinSourceSet.isTestModule
+    }
 
     override var actualPlatforms: KotlinPlatformContainer = defaultPlatform
         internal set
 
-    override var isTestModule: Boolean = defaultIsTestModule
+    override var isTestModule: Boolean = false
         internal set
 
     override fun toString() = name